24. SQL: Database interface¶
A lot of applications need to provide persistence for their data (or a part of it). This means the data needs to be somehow saved on the disk, to be read and manipulated later, possibly after the application has been terminated and restarted. Although Ada provides various solutions for this (including the use of the streams as declared in the Ada Reference Manual), the common technics is through the use of relational database management systems (RDBMS. The term database is in fact overloaded in this context, and has come to mean different things:
The software system that implements file and query management. This is generally provided by a third-party. The common abbreviation for these is DBMS. Queries are generally written in a language called SQL. One of the issues is that each DBMS tends to make minor changes to this language. Another issue is that the way to send these SQL commands to the DBMS is vendor-specific. GNATColl tries to abstract this communication through its own API. Optional components can instantiate this framework to specific DBMS, for example PostgreSQL and sqlite. Common API makes it relatively easy to change between two systems. For instance, development could be done using a local sqlite DBMS, and then deployed (after testing, of course!) on a PostgreSQL system.
The code in GNATColl is such that adding support for a new DBMS should be relatively easy.
A place where an application stores its data. The term database in this document refers to this meaning. In a relational database, this place is organized into tables, each of which contains a number of fields. A row in a table represents one object. The set of tables and their fields is called the schema of the database.
Traditionally, writing the SQL queries is done inline: special markers are inserted into your code to delimit sections that contain SQL code (as opposed to Ada code), and these are then preprocessed to generate actual code. This isn’t the approach chosen in GNATColl: there are several drawbacks, in particular your code is no longer Ada and various tools will choke on it.
The other usual approach is to write the queries as strings, which are passed, via a DBMS-specific API, to the DBMS server. This approach is very fragile:
The string might not contain well-formed SQL. This will unfortunately only be detected at run time when the DBMS complains.
This is not type safe. You might be comparing a text field with an integer, for instance. In some cases, the DBMS will accept that (sqlite for instance), but in some other cases it won’t (PostgreSQL). The result might then either raise an error, or return an empty list.
There is a risk of SQL injection. Assuming the string is constructed dynamically (using Ada’s & operator), it might be easy for a user to pass a string that breaks the query, and even destroys things in the database.
As discussed previously, the SQL code might not be portable across DBMS. For instance, creating an automatically increment integer primary key in a table is DBMS specific.
The string is fragile if the database schema changes. Finding whether a schema change impacts any of the queries requires looking at all the strings in your application.
performance might be an issue. Whenever you execute a query, the DBMS will analyze it, decide how to execute it (for instance, whether it should traverse all the rows of a table, or whether it can do a faster lookup), and then retrieve the results. The analysis pass is typically slow (relatively the overall execution time), and queries can in fact be prepared on the server: they are then analyzed only once, and it is possible to run them several times without paying the price of the analysis every time. Such a query can also be parameterized, in that some values can be deferred until the query is actually executed. All the above is made easy and portable in GNATColl, instead of requiring DBMS-specific techniques.
This might require large amount of code to setup the query, bind the parameters, execute it, and traverse the list of results.
GNATColl attempts to solve all these issues. It also provides further performance improvements, for instance by keeping connections to the DBMS open and reusing them when possible. A paper was published at the Ada-Europe conference in 2008 which describes the various steps we went through in the design of this library.
24.1. Database Abstraction Layers¶
GNATColl organizes the API into several layers, each written on top of the previous one and providing more abstraction. All layers are compatible with each other and can be mixed inside a given application.
This API is DBMS-specific, and is basically a mapping of the C API provided by the DBMS vendors into Ada. If you are porting C code, or working with an existing application, as a way to start using GNATColl before moving to higher levels of abstraction.
The code is found in
gnatcoll-sql-postgres-gnade.ads. The gnade part in the file names indicate that this code was initially inspired by the GNADE library that used to be available on the internet. Part of the code might in fact come from that library.
Using this API requires writing the SQL queries as strings, with all the disadvantages that were highlighted at the beginning of this chapter.
GNATCOLL.SQL and GNATCOLL.SQL.Exec
The first of these packages makes it possible to write type-safe queries strongly linked to the database schema (thus with a guarantee that the query is up-to-date with regards to the schema). To accomplish this, it also relies on code that is generated automatically from a description of your database schema, using the tool gnatcoll_db2ada. To simplify memory management, the queries are automatically referenced counted and freed when they are no longer needed.
The second of these packages provides communication with the DBMS. It provides a vendor-neutral API. You can send your queries either as strings, or preferably as written with GNATCOLL.SQL. It also provides a simple way to prepare parameterized statements on the server for maximum efficiency, as well as the reuse of existing DBMS connections. It provides a simple API to retrieve and manipulate the results from a query.
GNATCOLL.SQL.ORM and GNATCOLL.SQL.Sessions
This is an Object-Relational Mapping (ORM).
The first of these packages makes it possible to manipulate a database without writing SQL. Instead, you manipulate Ada objects (tagged types), whose primitive operations might transparently execute SQL queries. This API provides caching for maximum efficiency. It relies on code automatically generated by gnatcoll_db2ada from the schema of your database. The generated objects can then be extended in your own code if needed.
The second package encapsulates DBMS connections into higher-level objects which provide their own caching and work best with the ORM objects. A session is automatically released to a pool when no longer needed and will be reused later on.
The following sections will ignore the lower layer, and concentrate on the other layers. They share a number of types and, again, are fully compatible with each other. You could connect to the database, and then write some queries using GNATCOLL.SQL and some using GNATCOLL.SQL.ORM.
24.2. Database example¶
This section describes an example that will be extended throughout this chapter. We will build an application that represents a library. Such a library contains various media (books and DVDs for instance), and customers. A customer can borrow multiple media at the same time, but a media is either at a customer’s, or still in the library.
The GNATColl distribution includes an example directory which contains all the code and data for this example.
24.3. Database schema¶
As was mentioned earlier (Database Abstraction Layers), GNATColl relies on automatic code generation to provide a type safe interface to your database. This code is generated by an external tool called gnatcoll_db2ada provided as an optional component. In some cases, this tool requires an installation of python (www.python.org) on your machine, since part of the code is written in that language.
This tool is able to output various kind of information, and is fully described in the corresponding component. However, the input is always the same: this is the schema of your database, that is the list of tables and fields that make up your database. There exist two ways to provide that information:
From a running database
If you pass the DBMS vendor (postgresql, sqlite,…) and the connection parameters to gnatcoll_db2ada, it is able to query the schema on its own. However, this should not be the preferred method: this is similar to reverse engineering assembly code into the original high-level code, and some semantic information will be missing. For instance, in SQL we have to create tables just to represent the many-to-many relationships. These extra tables are part of the implementation of the schema, but are just noise when it comes to the semantics of the schema. For this reason, it is better to use the second solution below:
From a textual description
Using the -dbmodel switch to gnatcoll_db2ada, you can pass a file that describes the schema. We do not use SQL as the syntax in this, because as explained above this is too low-level. This text file also provides additional capabilities that do not exist when reverse-engineering an existing database, for instance the ability to use name to represent reverse relationships for foreign keys (see below and the ORM).
The most convenient editor for this file is Emacs, using the org-mode which provides convenient key shortcuts for editing the contents of ASCII tables. But any text editor will do, and you do not need to align the columns in this file.
All lines starting with a hash sign (‘#’) will be ignored.
This file is a collection of ASCII tables, each of which relates to one table or one SQL view in your database. The paragraphs start with a line containing:
table ::= '|' ('ABSTRACT')? ('TABLE'|'VIEW') ['(' supertable ')'] '|' <name> '|' <name_row>
“name” is the name of the table. The third pipe and third column are optional, and should be used to specify the name for the element represented by a single row. For instance, if the table is called “books”, the third column could contain “book”. This is used when generating objects for use with GNATCOLL.SQL.ORM.
If the first line starts with the keyword ABSTRACT, then no instance of that table actually exists in the database. This is used in the context of table inheritance, so define shared fields only once among multiple tables.
The keyword TABLE can be followed by the name of a table from which it inherits the fields. Currently, that supertable must be abstract, and the fields declared in that table are simply duplicated in the new table.
Following the declaration of the table, the file then describe their fields, each on a separate line. Each of these lines must start with a pipe character (“|”), and contain a number of pipe-separated fields. The order of the fields is always given by the following grammar:
fields ::= '|' <name> '|' <type> '|' ('PK'|''|'NULL'|'NOT NULL'|'INDEX'|'UNIQUE'|'NOCASE') '|' [default] '|' [doc] '|'
The type of the field is the SQL type (“INTEGER”, “TEXT”, “TIMESTAMP”, “DATE”, “DOUBLE PRECISION”, “MONEY”, “BOOLEAN”, “TIME”, “CHARACTER(1)”). Any maximal length can be specified for strings, not just 1 as in this example. The tool will automatically convert these to Ada when generating Ada code. A special type (“AUTOINCREMENT”) is an integer that is automatically incremented according to available ids in the table. The exact type used will depend on the specific DBMS.
The property ‘NOCASE’ indicates that comparison should be case insensitive for this field.
If the field is a foreign key (that is a value that must correspond to a row in another table), you can use the special syntax for its type:
fk_type ::= 'FK' <table_name> [ '(' <reverse_name> ')' ]
As you can see, the type of the field is not specified explicitly, but will always be that of the foreign table’s primary key. With this syntax, the foreign table must have a single field for its primary key. GNATColl does not force a specific order for the declaration of tables: if is valid to have a foreign key to a table that hasn’t been declared yet. There is however a restriction if you use the model to create a sqlite database (through the -createdb switch of gnatcoll_db2ada): in this case, a reference to a table that hasn’t been defined yet may not be not through a field marked as NOT NULL. This is a limitation of the sqlite backend itself. The solution in this case is to reorder the declaration of tables, or drop the NOT NULL constraint.
Another restriction is that a foreign key that is also a primary key must reference a table that has already been defined. You need to reorder the declaration of your tables to ensure this is the case.
“reverse_name” is the optional name that will be generated in the Ada code for the reverse relationship, in the context of GNATCOLL.SQL.ORM. If the “reverse_name” is empty (the parenthesis are shown), no reverse relationship is generated. If the parenthesis and the reverse_name are both omitted, a default name is generated based on the name of the field.
The third column in the fields definition indicates the constraints of the type. Multiple keywords can be used if they are separated by commas. Thus, “NOT NULL, INDEX” indicates a column that must be set by the user, and for which an index is created to speed up look ups.
A primary key (“PK”)
The value must be defined (“NOT NULL”)
The value can be left undefined (“NULL”)
A unique constraint and index (“UNIQUE”)
An index should be created for that column (“INDEX”) to speed up the lookups.
The automatic index created for a Foreign Key should not be created (“NOINDEX”). Every time a field references another table, GNATColl will by default create an index for it, so that the ORM can more efficiently do a reverse query (from the target table’s row find all the rows in the current table that reference that target row). This will in general provide more efficiency, but in some cases you never intend to do the reverse query and thus can spare the extra index.
The fourth column gives the default value for the field, and is given in SQL syntax. Strings must be quoted with single quotes.
The fifth column contains documentation for the field (if any). This documentation will be included in the generated code, so that IDEs can provide useful tooltips when navigating your application’s code.
After all the fields have been defined, you can specify extract constraints on the table. In particular, if you have a foreign key to a table that uses a tuple as its primary key, you can define that foreign key on a new line, as:
FK ::= '|' "FK:" '|' <table> '|' <field_names>* '|' <field_names>* '|'
| TABLE | tableA | | FK: | tableB | fieldA1, fieldA2 | fieldB1, fieldB2 |
It is also possible to create multi-column indexes, as in the following example. In this case, the third column contains the name of the index to create. If left blank, a default name will be computed by GNATColl:
| TABLE | tableA | | INDEX: | field1,field2,field3 | name |
The same way the unique multi-column constraint and index can be created. The name is optional.TABLE | tableA |UNIQUE: | field1,field2,field3 | name |
Going back to the example we described earlier (Database example), let’s describe the tables that are involved.
The first table contains the customers. Here is its definition:
| TABLE | customers | customer || The customer for the library | | id | AUTOINCREMENT | PK || Auto-generated id | | first | TEXT | NOT NULL || Customers' first name | | last | TEXT | NOT NULL, INDEX || Customers' last name |
We highly recommend to set a primary key on all tables. This is a field whose value is unique in the table, and thus that can act as an identifier for a specific row in the table (in this case for a specific customer). We recommand using integers for these ids for efficiency reasons. It is possible that the primary key will be made of several fields, in which case they should all have the “PK” constraint in the third column.
A table with no primary key is still usable. The difference is in the code generated for the ORM (The Object-Relational Mapping layer (ORM)), since the Delete operation for this table will raise a Program_Error instead of doing the actual deletion (that’s because there is no guaranteed unique identifier for the element, so the ORM does not know which one to delete – we do not depend on having unique internal ids on the table, like some DBMS have). Likewise, the elements extracted from such a primary key-less table will not be cached locally in the session, and cannot be updated (only new elements can be created in the table).
As we mentioned, the library contains two types of media, books and DVDs. Each of those has a title, an author. However, a book also has a number of pages and a DVD has a region where it can be viewed. There are various ways to represent this in a database. For illustration purposes, we will use table inheritance here: we will declare one abstract table (media) which contains the common fields, and two tables to represent the types of media.
As we mentioned, a media can be borrowed by at most one customer, but a customer can have multiple media at any point in time. This is called a one-to-many relationship. In SQL, this is in general described through the use of a foreign key that goes from the table on the “many” side. In this example, we therefore have a foreign key from media to customers. We also provide a name for the reverse relationship, which will become clearer when we describe the ORM interface.
Here are the declarations:
| ABSTRACT TABLE | media | media || The contents of the library | | id | AUTOINCREMENT | PK || Auto-generated id | | title | TEXT | || The title of the media | | author | TEXT | || The author | | published | DATE | || Publication date | | borrowed_by | FK customers(items) | NULL || Who borrowed the media | | TABLE (media) | books | book | | The books in the library | | pages | INTEGER | | 100 | | | TABLE (media) | dvds | dvd | | The dvds in the library | | region | INTEGER | | 1 | |
For this example, all this description is put in a file called
24.4. Connecting to the database¶
This library abstracts the specifics of the various database engines it supports. Ideally, code written for one database could be ported almost transparently to another engine. This is not completely doable in practice, since each system has its own SQL specifics, and unless you are writing things very carefully, the interpretation of your queries might be different from one system to the next.
However, the Ada code should remain untouched if you change the engine. Various engines are supported out of the box (PostgreSQL and Sqlite), although new ones can be added by overriding the appropriate SQL type (Database_Connection). When you compile GNATColl, the build scripts will try and detect what systems are installed on your machine, and only build support for those. It is possible, if no database was installed on your machine at that time, that the database interface API is available (and your application compiles), but no connection can be done to database at run time.
To connect to a DBMS, you need to specify the various connection parameters. This is done via a GNATCOLL.SQL.Exec.Database_Description object. The creation of this object depends on the specific DBMS you are connecting to (and this is the only part of your code that needs to know about the specific system). The packages GNATCOLL.SQL.Postgres and GNATCOLL.SQL.Sqlite contain a Setup function, whose parameters depend on the DBMS. They provide full documentation for their parameters. Let’s take a simple example from sqlite:
with GNATCOLL.SQL.Sqlite; -- or Postgres declare DB_Descr : GNATCOLL.SQL.Exec.Database_Description; begin DB_Descr := GNATCOLL.SQL.Sqlite.Setup ("dbname.db"); end
At this point, no connection to the DBMS has been done, and no information was exchanged.
To communicate with the database, however, we need to create another object, a GNATCOLL.SQL.Exec.Database_Connection. Your application can create any number of these. Typically, one would create one such connection per task in the application, although other strategies are possible (like a pool of reusable connections, where a task might be using two connections and another task none at any point in time).
If you do not plan on using the ORM interface from GNATCOLL.SQL.ORM, GNATColl provides a simple way to create a task-specific connection. While in this task, the same connection will always be returned (thus you do not have to pass it around in parameter, although the latter might be more efficient):
declare DB : GNATCOLL.SQL.Exec.Database_Connection; begin DB := GNATCOLL.SQL.Exec.Get_Task_Connection (Description => DB_Descr); end;
If your application is not multi-tasking, or you wish to implement your own strategy for a connection pool, you can also use the following code (using Ada 2005 dotted notation when calling the primitive operation). This code will always create a new connection, not reuse an existing one, as opposed to the code above:
declare DB : GNATCOLL.SQL.Exec.Database_Connection; begin DB := DB_Descr.Build_Connection; end;
A note on concurrency: if you implement your own pool, you might sometimes end up with dead locks when using sqlite. If a task uses two or more connections to sqlite, and you setup GNATCOLL to create SQL transactions even for SELECT statements (see GNATCOLL.SQL.Sqlite.Always_Use_Transactions), the following scenario will result in a deadlock:
DB1 := ... new connection to sqlite ... execute a SELECT through DB1. The latter then holds a shared ... lock, preventing other connections from writing (but not from ... reading). DB2 := ... another connection in the same thread ... execute an INSERT through DB2. This tries to get a lock, which ... will fail while DB1 holds the shared lock. Since these are in ... the same thread, this will deadlock.
By default, GNATCOLL will not create SQL transactions for select statements to avoid this case, which occurs frequently in code.
If you wish to reuse an existing connection later on, you must reset it. This terminates any on-going SQL transaction, and resets various internal fields that describe the state of the connection:
In all three cases, the resulting database connection needs to be freed when you no longer needed (which might be when your program terminates if you are using pools) to avoid memory leaks. Nothing critical will appear if you do not close, though, because the transactions to the DBMS server are saved every time you call Commit in any case. So the code would end with:
Free (DB); -- for all connections you have opened Free (DB_Descr);
At this point, there still hasn’t been any connection to the DBMS. This will be done the first time a query is executed. If for some reason the connection to the DBMS server is lost, GNATColl will automatically attempt to reconnect a number of times before it gives up. This might break if there was an ongoing SQL transaction, but simplifies your code since you do not have to handle reconnection when there was a network failure, for instance.
As we saw before, the database interface can be used in multi-tasking applications. In such a case, it is recommended that each thread has its own connection to the database, since that is more efficient and you do not have to handle locking. However, this assumes that the database server itself is thread safe, which most often is the case, but not for sqlite for instance. In such a case, you can only connect one per application to the database, and you will have to manage a queue of queries somehow.
If you want to use GNATCOLL.SQL.Sessions along with the Object-Relational Mapping API, you will need to initialize the connection pool with the Database_Description, but the session will then take care automatically of creating the Database_Connection. See later sections for more details.
24.5. Loading initial data in the database¶
We have now created an empty database. To make the queries we will write later more interesting, we are going to load initial data.
There are various ways to do it:
Manually or with an external tool
One can connect to the database with an external tool (a web interface when the DBMS provides one for instance), or via a command line tool (psql for PostgreSQL or sqlite3 for Sqlite), and start inserting data manually. This shows one of the nice aspects of using a standard DBMS for your application: you can alter the database (for instance to do minor fixes in the data) with a lot of external tools that were developed specifically for that purpose and that provide a nice interface. However, this is also tedious and error prone, and can’t be repeat easily every time we recreate the database (for instance before running automatic tests).
As we will describe later, GNATColl contains all the required machinery for altering the contents of the database and creating new objects. Using GNATCOLL.SQL.ORM this can also be done at a high-level and completely hide SQL.
Loading a data file
A lot of frameworks call such a file that contains initial data a “fixture”. We will use this techniques as an example. At the Ada level, this is a simple call to GNATCOLL.SQL.Inspect.Load_Data. The package contains a lot more than just this subprogram (The_gnatcoll_db2ada_tool):
declare File : GNATCOLL.VFS.Virtual_File := Create ("fixture.txt"); DB : Database_Connection; -- created earlier begin GNATCOLL.SQL.Inspect.Load_Data (DB, File); DB.Commit; end;
The format of this file is described just below.
As we mentioned, GNATColl can load data from a file. The format of this file is similar to the one that describes the database schema. It is a set of ASCII tables, each of which describes the data that should go in a table (it is valid to duplicate tables). Each block starts with two lines: The first one has two mandatory columns, the first of which contains the text “TABLE”, and the second contains the name of the table you want to fill. The second line should contain as many columns as there are fields you want to set. Not all the fields of the table need to have a corresponding column if you want to set their contents to NULL (provided, of course, that your schema allows it). For instance, we could add data for our library example as such:
| TABLE | customers | | | id | first | last | |-------+-----------+--------| | 1 | John | Smith | | 2 | Alain | Dupont | | TABLE | books | | | | | title | author | pages | published | borrowed_by | |------------+---------+-------+------------+-------------| | Art of War | Sun Tzu | 90 | 01-01-2000 | 1 | | Ada RM | WRG | 250 | 01-07-2005 | |
A few comments on the above: the id for books is not specified, although the column is the primary key and therefore cannot be NULL. In fact, since the type of the id was set to AUTOINCREMENT, GNATColl will automatically assign valid values. We did not use this approach for the id of customers, because we need to know this id to set the borrowed_by field in the books table.
There is another approach to setting the borrowed_by field, which is to give the value of another field of the customers table. This of course only work if you know this value is unique, but that will often be the case in your initial fixtures. Here is an example:
| TABLE | dvds | | | | title | author | region | borrowed_by(&last) | |--------------+-----------+--------+--------------------| | The Birds | Hitchcock | 1 | &Smith | | The Dictator | Chaplin | 3 | &Dupont |
Here, the title of the column indicates that any value in this column might be a reference to the customers.last value. Values which start with an ampersand (“&”) will therefore be looked up in customers.last, and the id of the corresponding customer will be inserted in the dvds table. It would still be valid to use directly customer ids instead of references, this is just an extra flexibility that the references give you to make your fixtures more readable.
However, if we are using such references we need to provide the database schema to Load_Data so that it can write the proper queries. This is done by using other services of the GNATCOLL.SQL.Inspect package.
The code for our example would be:
Load_Data (DB, Create ("fixture.txt"), New_Schema_IO (Create ("dbschema.txt")).Read_Schema);
24.6. Writing queries¶
The second part of the database support in GNATColl is a set of Ada subprograms which help write SQL queries. Traditional ways to write such queries have been through embedded SQL (which requires a preprocessing phase and complicates the editing of source files in Ada-aware editors), or through simple strings that are passed as is to the server. In the latter case, the compiler can not do any verification on the string, and errors such a missing parenthesis or misspelled table or field names will not be detected until the code executes the query.
GNATColl tries to make sure that code that compiles contains syntactically correct SQL queries and only reference existing tables and fields. This of course does not ensure that the query is semantically correct, but helps detect trivial errors as early as possible.
Such queries are thus written via calls to Ada subprograms, as in the following example:
with GNATCOLL.SQL; use GNATCOLL.SQL; with Database; use Database; declare Q : SQL_Query; begin Q := SQL_Select (Fields => Max (Ticket_Priorities.Priority) & Ticket_Priorities.Category, From => Ticket_Priorities, Where => Ticket_Priorities.Name /= "low", Group_By => Ticket_Priorities.Category); end;
The above example will return, for each type of priority (internal or customer) the highest possible value. The interest of this query is left to the user…
This is very similar to an actual SQL query. Field and table names come from the package that was automatically generated by the gnatcoll_db2ada tool, and therefore we know that our query is only referencing existing fields. The syntactic correctness is ensured by standard Ada rules. The SQL_Select accepts several parameters corresponding to the usual SQL attributes like GROUP BY, HAVING, ORDER BY and LIMIT.
The From parameter could be a list of tables if we need to join them in some ways. Such a list is created with the overridden “&” operator, just as for fields which you can see in the above example. GNATColl also provides a Left_Join function to join two tables when the second might have no matching field (see the SQL documentation).
Similar functions exist for SQL_Insert, SQL_Update and
SQL_Delete. Each of those is extensively documented in the
It is worth noting that we do not have to write the query all at once. In fact, we could build it depending on some other criteria. For instance, imagine we have a procedure that does the query above, and omits the priority specified as a parameter, or shows all priorities if the empty string is passed. Such a procedure could be written as:
procedure List_Priorities (Omit : String := "") is Q : SQL_Query; C : SQL_Criteria := No_Criteria; begin if Omit /= "" then C := Ticket_Priorities.Name /= Omit; end if; Q := SQL_Select (Fields => ..., -- as before Where => C); end;
With such a code, it becomes easier to create queries on the fly than it would be with directly writing strings.
The above call has not sent anything to the database yet, only created a data structure in memory (more precisely a tree). In fact, we could be somewhat lazy when writing the query and rely on auto-completion, as in the following example:
Q := SQL_Select (Fields => Max (Ticket_Priorities.Priority) & Ticket_Priorities.Category, Where => Ticket_Priorities.Name /= "low"); Auto_Complete (Q);
This query is exactly the same as before. However, we did not have to specify the list of tables (which GNATColl can compute on its own by looking at all the fields referenced in the query), nor the list of fields in the GROUP BY clause, which once again can be computed automatically by looking at those fields that are not used in a SQL aggregate function. This auto-completion helps the maintenance of those queries.
There is another case where GNATColl makes it somewhat easier to write the queries, and that is to handle joins between tables. If your schema was build with foreign keys, GNATColl can take advantage of those.
Going back to our library example, let’s assume we want to find out all the books that were borrowed by the user “Smith”. We need to involve two tables (Books and Customers), and provide a join between them so that the DBMS knows how to associate the rows from one with the rows from the other. Here is a first example for such a query:
Q := SQL_Select (Fields => Books.Title & Books.Pages, From => Books & Customers, Where => Books.Borrowed_By = Customers.Id and Customers.Last = "Smith");
In fact, we could also use auto-completion, and let GNATColl find out the involved tables on its own. We thus write the simpler:
Q := SQL_Select (Fields => Books.Title & Books.Pages, Where => Books.Borrowed_By = Customers.Id and Customers.Last = "Smith");
There is one more things we can do to simplify the query and make it more solid if the schema of the database changes. For instance, when a table has a primary key made up of several fields, we need to make sure we always have an “=” statement in the WHERE clause for all these fields between the two tables. In our example above, we could at some point modify the schema so that the primary key for customers is multiple (this is unlikely in this example of course). To avoid this potential problems and make the query somewhat easier to read, we can take advantage of the FK subprograms generated by gnatcoll_db2ada. Using the Ada05 dotted notation for the call, we can thus write:
Q := SQL_Select (Fields => Books.Title & Books.Pages, Where => Books.FK (Customers) and Customers.Last = "Smith");
Regarding memory management, there is no need for explicitly freeing memory in the above code. GNATColl will automatically do this when the query is no longer needed.
24.7. Executing queries¶
Once we have our query in memory, we need to pass it on to the database server itself, and retrieve the results.
Executing is done through the GNATCOLL.SQL.Exec package, as in the following example:
declare R : Forward_Cursor; begin R.Fetch (Connection => DB, Query => Q); end;
This reuses the connection we have established previously (DB) (although now we are indeed connecting to the DBMS for the first time) and sends it the query. The result of that query is then stored in R, to be used later.
Some SQL commands execute code on the DBMS, but do not return a result. In this case, you can use Execute instead of Fetch. This is the case when you execute an INSERT or UPDATE statement for instance. Using Execute avoids the need to declare the local variable R.
If for some reason the connection to the database is no longer valid (a transient network problem for instance), GNATColl will attempt to reconnect and re-execute your query transparently, so that your application does not need to handle this case.
We’ll describe later (Getting results) how to analyze the result of the query.
Some versions of Fetch have an extra parameter Use_Cache, set to False by default. If this parameter is true, and the exact same query has already been executed before, its result will be reused without even contacting the database server. The cache is automatically invalidated every hour in any case. This cache is mostly useful for tables that act like enumeration types. In this case, the contents of the table changes very rarely, and the cache can provide important speedups, whether the server is local or distant. However, we recommend that you do actual measurements to know whether this is indeed beneficial for you. You can always invalidate the current cache with a call to Invalidate_Cache to force the query to be done on the database server.
If your query produces an error (whether it is invalid, or any other reason), a flag is toggled in the Connection parameter, which you can query through the Success subprogram. As a result, a possible continuation of the above code is:
if Success (DB) then ... else ... -- an error occurred end if
GNATColl also tries to be helpful in the way it handles SQL transactions. Such transactions are a way to execute your query in a sandbox, i.e. without affecting the database itself until you decide to COMMIT the query. Should you decide to abort it (or ROLLBACK as they say for SQL), then it is just as if nothing happened. As a result, it is in general recommended to do all your changes to the database from within a transaction. If one of the queries fail because of invalid parameters, you just rollback and report the error to the user. The database is still left in a consistent state. As an additional benefit, executing within a transaction is sometimes faster, as is the case for PostgreSQL for instance.
To help with this, GNATColl will automatically start a transaction the first time you edit the database. It is then your responsibility to either commit or rollback the transaction when you are done modifying. A lot of database engines (among which PostgreSQL) will not accept any further change to the database if one command in the transaction has failed. To take advantage of this, GNATColl will therefore not even send the command to the server if it is in a failure state.
Here is code sample that modifies the database:
Execute (DB, SQL_Insert (...)); -- Executed in the same transaction Commit_Or_Rollback (DB); -- Commit if both insertion succeeded, rollback otherwise -- You can still check Success(DB) afterward if needed
24.8. Prepared queries¶
The previous section showed how to execute queries and statements. But these were in fact relatively inefficient.
With most DBMS servers, it is possible to compile the query once on the server, and then reuse that prepared query to significantly speed up later searches when you reuse that prepared statement.
It is of course pretty rare to run exactly the same query or statement multiple times with the same values. For instance, the following query would not give much benefit if it was prepared, since you are unlikely to reuse it exactly as is later on:
SELECT * FROM data WHERE id=1
SQL (and GNATColl) provide a way to parameterize queries. Instead of hard-coding the value 1 in the example above, you would in fact use a special character (unfortunately specific to the DBMS you are interfacing to) to indicate that the value will be provided when the query is actually executed. For instance, sqlite would use:
SELECT * FROM data WHERE id=?
You can write such a query in a DBMS-agnostic way by using GNATColl.
Assuming you have automatically generated
database.ads by using
gnatcoll_db2ada, here is the corresponding Ada code:
with Database; use Database; Q : constant SQL_Query := SQL_Select (Fields => Data.Id & Data.Name From => Data, Where => Data.Id = Integer_Param (1));
GNATColl provides a number of functions (one per type of field) to indicate that the value is currently unbound. Integer_Param, Text_Param, Boolean_Param,… All take a single argument, which is the index of the corresponding parameter. A query might need several parameters, and each should have a different index. On the other hand, the same parameter could be used in several places in the query.
Although the query above could be executed as is by providing the values for the parameters, it is more efficient, as we mentioned at the beginning, to compile it on the server. In theory, this preparation is done within the context of a database connection (thus cannot be done for a global variable, where we do not have connections yet, and where the query might be executed by any connection later on).
GNATColl will let you indicate that the query should be prepared. This basically sets up some internal data, but does not immediately compile it on the server. The first time the query is executed in a given connection, though, it will first be compiled. The result of this compilation will be reused for that connection from then on. If you are using a second connection, it will do its own compilation of the query.
So in our example we would add the following global variable:
P : constant Prepared_Statement := Prepare (Q, On_Server => True);
Two comments about this code:
You do not have to use global variables. You can prepare the statement locally in a subprogram. A Prepared_Statement is a reference counted type, that will automatically free the memory on the server when it goes out of scope.
Here, we prepared the statement on the server. If we had specified On_Server => False, we would still have sped things up, since Q would be converted to a string that can be sent to the DBMS, and from then on reused that string (note that this conversion is specific to each DBMS, since they don’t always represent things the same way, in particular parameters, as we have seen above). Thus every time you use P you save the time of converting from the GNATColl tree representation of the query to a string for the DBMS.
Now that we have a prepared statement, we can simply execute it. If the statement does not require parameters, the usual Fetch and Execute subprograms have versions that work exactly the same with prepared statements. They also accept a Params parameter that contains the parameter to pass to the server. A number of “+” operators are provided to create those parameters:
declare F : Forward_Cursor; begin F.Fetch (DB, P, Params => (1 => +2)); F.Fetch (DB, P, Params => (1 => +3)); end;
Note that for string parameters, the “+” operator takes an access to a string. This is for efficiency, to avoid allocating memory and copying the string, and is safe because the parameters are only needed while Fetch executes (even for a Forward_Cursor.
Back to our library example. We showed earlier how to write a query that retrieves the books borrowed by customer “Smith”. We will now make this query more general: given a customer name, return all the books he has borrowed. Since we expect to use this often, we will prepare it on the server (in real life, this query is of little interest since the customer name is not unique, we would instead use a query that takes the id of the customer). In general we would create a global variable with:
Borrowed : constant Prepared_Statement := Prepare (SQL_Select (Fields => Books.Title & Books.Pages, Where => Books.FK (Customers) and Customers.Last = Text_Param (1)); Auto_Complete => True, On_Server => True);
Then when we need to execute this query, we would do:
declare Name : aliased String := "Smith"; begin R.Fetch (DB, Borrowed, Params => (1 => +Smith'Access)); end;
There is one last property on Prepared_Statement`s: when you prepare them, you can pass a `Use_Cache => True parameter. When this is used, the result of the query will be cached by GNATColl, and reuse when the query is executed again later. This is the fastest way to get the query, but should be used with care, since it will not detect changes in the database. The local cache is automatically invalidated every hour, so the query will be performed again at most one hour later. Local caching is disabled when you execute a query with parameters. In this case, prepare the query on the server which will still be reasonably fast.
Finally, here are some examples of timings. The exact timing are irrelevant, but it is interesting to look at the different between the various scenarios. Each of them performs 100_000 simple queries similar to the one used in this section:
Not preparing the query, using `Direct_Cursor`: 4.05s Not preparing the query, using `Forward_Cursor`, and only retrieving the first row: 3.69s Preparing the query on the client (`On_Server => False`), with a `Direct_Cursor`. This saves the whole `GNATCOLL.SQL` manipulations and allocations: 2.50s Preparing the query on the server, using `Direct_Cursor`: 0.55s Caching the query locally (`Use_Cache => True`): 0.13s
24.9. Getting results¶
Once you have executed a SELECT query, you generally need to examine the rows that were returned by the database server. This is done in a loop, as in:
while Has_Row (R) loop Put_Line ("Max priority=" & Integer_Value (R, 0)'Img & " for category=" & Value (R, 1)); Next (R); end loop;
You can only read one row at a time, and as soon as you have moved to the next row, there is no way to access a previously fetched row. This is the greatest common denominator between the various database systems. In particular, it proves efficient, since only one row needs to be kept in memory at any point in time.
For each row, we then call one of the Value or *Value functions which return the value in a specific row and a specific column.
We mentioned earlier there was no way to go back to a row you fetched previously except by executing the query again. This is in fact only true if you use a Forward_Cursor to fetch the results.
But GNATColl provides another notion, a Direct_Cursor. In this case, it fetches all the rows in memory when the query executes (thus it needs to allocate more memory to save every thing, which can be costly if the query is big). This behavior is supported natively by PostgreSQL, but doesn’t exist with sqlite, so GNATColl will simulate it as efficiently as possible. But it will almost always be faster to use a Forward_Cursor.
In exchange for this extra memory overhead, you can now traverse the list of results in both directions, as well as access a specific row directly. It is also possible to know the number of rows that matched (something hard to do with a Forward_Cursor since you would need to traverse the list once to count, and then execute the query again if you need the rows themselves).
Direct_Cursor, produced from prepeared statements, could be indexed by the specified field value and routine Find could set the cursor position to the row with specified field value.:
-- Prepared statement should be declared on package level. Stmt : Prepared_Statement := Prepare ("select Id, Name, Address from Contact order by Name" Use_Cache => True, Index_By => Field_Index'First); procedure Show_Contact (Id : Integer) is CI : Direct_Cursor; begin CI.Fetch (DB, Stmt); CI.Find (Id); -- Find record by Id if CI.Has_Row then Put_Line ("Name " & CI.Value (1) & " Address " & CI.Value (2)); else Put_Line ("Contact id not found."); end if; end Show_Contact;
In general, the low-level DBMS C API use totally different approaches for the two types of cursors (when they even provide them). By contrast, GNATColl makes it very easy to change from one to the other just by changing the type of a the result variable. So you would in general start with a Forward_Cursor, and if you discover you in fact need more advanced behavior you can pay the extra memory cost and use a Direct_Cursor.
For both types of cursors, GNATColl automatically manages memory (both on the client and on the DBMS), thus providing major simplification of the code compared to using the low-level APIs.
24.10. Creating your own SQL types¶
GNATColl comes with a number of predefined types that you can use in
gnatcoll_db2ada will generate a file using any of these
predefined types, based on what is defined in your actual database.
But sometimes, it is convenient to define your own SQL types to better represent the logic of your application. For instance, you might want to define a type that would be for a Character field, rather than use the general SQL_Field_Text, just so that you can write statements like:
declare C : Character := 'A'; Q : SQL_Query; begin Q := SQL_Select (.., Where => Table.Field = C); end
This is fortunately easily achieved by instantiating one generic package, as such:
with GNATCOLL.SQL_Impl; use GNATCOLL.SQL_Impl; function To_SQL (C : Character) return String is begin return "'" & C & "'"; end To_SQL; package Character_Fields is new Field_Types (Character, To_SQL); type SQL_Field_Character is new Character_Fields.Field with null record;
This automatically makes available both the field type (which you can use in
your database description, as
gnatcoll_db2ada would do, but also
all comparison operators like <, >, =, and so on, both
to compare with another character field, or with Character Ada
variable. Likewise, this makes available the assignment operator =
so that you can create INSERT statements in the database.
Finally, the package Character_Fields contain other generic packages which you can instantiate to bind SQL operators and functions that are either predefined in SQL and have no equivalent in GNATColl yet, or that are functions that you have created yourself on your DBMS server.
See the specs of GNATCOLL.SQL_Impl for more details. This package is only really useful when writing your own types, since otherwise you just have to use GNATCOLL.SQL to write the actual queries.
See also GNATCOLL.SQL_Fields for an example on how to have a full integration with other parts of GNATCOLL.SQL.
24.11. Query logs¶
In Traces: Logging information we discovered the logging module of GNATColl. The database interface uses this module to log the queries that are sent to the server.
If you activate traces in your application, the user can then activate one of the following trace handles to get more information on the exchange that exists between the database and the application. As we saw before, the output of these traces can be sent to the standard output, a file, the system logs,…
The following handles are provided:
SQL.ERROR This stream is activated by default. Any error returned by the database (connection issues, failed transactions,…) will be logged on this stream
SQL This stream logs all queries that are not SELECT queries, ie mostly all queries that actually modify the database
SQL.SELECT This stream logs all select queries. It is separated from SQL because very often you will be mostly interested in the queries that impact the database, and logging all selects can generate a lot of output.
In our library example, we would add the following code to see all SQL statements executed on the server:
with GNATCOLL.Traces; use GNATCOLL.Traces; procedure Main is begin GNATCOLL.Traces.Parse_Config_File (".gnatdebug"); ... -- code as before GNATCOLL.Traces.Finalize; -- reclaim memory
and then create a .gnatdebug in the directory from which we launch our executable. This file would contain a single line containing “+” to activate all log streams, or the following to activate only the subset of fields related to SQL:
SQL=yes SQL.SELECT=yes SQL.LITE=yes
24.12. Writing your own cursors¶
The cursor interface we just saw is low-level, in that you get access to each of the fields one by one. Often, when you design your own application, it is better to abstract the database interface layer as much as possible. As a result, it is often better to create record or other Ada types to represent the contents of a row.
Fortunately, this can be done very easily based on the API provided by GNATCOLL.SQL. Note that GNATCOLL.SQL.ORM provides a similar approach based on automatically generated code, so might be even better. But it is still useful to understand the basics of providing your own objects.
Here is a code example that shows how this can be done:
type Customer is record Id : Integer; First, Last : Unbounded_String; end record; type My_Cursor is new Forward_Cursor with null record; function Element (Self : My_Cursor) return My_Row; function Do_Query (DB, ...) return My_Cursor;
The idea is that you create a function that does the query for you (based on some parameters that are not shown here), and then returns a cursor over the resulting set of rows. For each row, you can use the Element function to get an Ada record for easier manipulation.
Let’s first see how these types would be used in practice:
declare C : My_Cursor := Do_Query (DB, ...); begin while Has_Row (C) loop Put_Line ("Id = " & Element (C).Id); Next (C); end loop; end;
So the loop itself is the same as before, except we no longer access each of the individual fields directly. This means that if the query changes to return more fields (or the same fields in a different order for instance), the code in your application does not need to change.
The specific implementation of the subprograms could be similar to the following subprograms (we do not detail the writing of the SQL query itself, which of course is specific to your application):
function Do_Query return My_Cursor is Q : constant SQL_Query := ....; R : My_Cursor; begin R.Fetch (DB, Q); return R; end Do_Query; function Element (Self : My_Cursor) return My_Row is begin return Customer' (Id => Integer_Value (Self, 0), First => To_Unbounded_String (Value (Self, 1)), Last => To_Unbounded_String (Value (Self, 2))); end Element;
There is one more complex case though. It might happen that an element needs access to several rows to fill the Ada record. For instance, if we are writing a CRM application and query the contacts and the companies they work for, it is possible that a contact works for several companies. The result of the SQL query would then look like this:
contact_id | company_id 1 | 100 1 | 101 2 | 100
The sample code shown above will not work in this case, since Element is not allowed to modify the cursor. In such a case, we need to take a slightly different approach:
type My_Cursor is new Forward_Cursor with null record; function Do_Query return My_Cursor; -- as before procedure Element_And_Next (Self : in out My_Cursor; Value : out My_Row);
where Element_And_Next will fill Value and call Next as many times as needed. On exit, the cursor is left on the next row to be processed. The usage then becomes:
while Has_Row (R) loop Element_And_Next (R, Value); end loop;
To prevent the user from using Next incorrectly, you should probably override Next with a procedure that does nothing (or raises a Program_Error maybe). Make sure that in Element_And_Next you are calling the inherited function, not the one you have overridden, though.
There is still one more catch. The user might depend on the two subprograms Rows_Count and Processed_Rows to find out how many rows there were in the query. In practice, he will likely be interested in the number of distinct contacts in the tables (2 in our example) rather than the number of rows in the result (3 in the example). You thus need to also override those two subprograms to return correct values.
24.13. The Object-Relational Mapping layer (ORM)¶
GNATColl provides a high-level interface to manipulate persistent objects stored in a database, using a common paradigm called an object-relational mapping. Such mappings exist for most programming languages. In the design of GNATColl, we were especially inspired by the python interface in django and sqlalchemy, although the last two rely on dynamic run time introspection and GNATColl relies on code generation instead.
This API is still compatible with GNATCOLL.SQL. In fact, we’ll show below cases where the two are mixed. It can also be mixed with GNATCOLL.SQL.Exec, although this might be more risky. Communication with the DBMS is mostly transparent in the ORM, and it uses various caches to optimize things and make sure that if you modify an element the next querie(s) will also return it. If you use GNATCOLL.SQL.Exec directly you are bypassing this cache so you risk getting inconsistent results in some cases.
In ORM, a table is not manipulated directly. Instead, you manipulate objects that are read or written to a table. When we defined our database schema (Database schema), we gave two names on the first line of a table definition. There was the name of the table in the database, and the name of the object that each row represent. So for our library example we have defined Customer, Book and Dvd objects. These objects are declared in a package generated automatically by gnatcoll_db2ada.
There is first one minor change we need to do to our library example. The ORM currently does not handle properly cases where an abstract class has foreign keys to other tables. So we remove the borrowed_by field from the Media table, and change the books table to be:
| TABLE (media) | books | book | | The books in the library | | pages | INTEGER | | 100 | | | borrowed_by | FK customers(borrowed_books) | NULL | | Who borrowed the media |
Let’s thus start by generating this code. We can replace the command we ran earlier (with the -api switch) with one that will also generate the ORM API:
gnatcoll_db2ada -dbmode dbschema.txt -api Database -orm ORM
The ORM provides a pool of database connections through the package
GNATCOLL.SQL.Sessions. A session therefore acts as a wrapper around
a connection, and provides a lot more advanced features that will be
described later. The first thing to do in the code is to configure the
session pool. The Setup procedure takes a lot of parameters to
make sessions highly configurable. Some of these parameters will be
described and used in this documentation, others are for special usage and
are only documented in
gnatcoll-sql-sessions.ads. Here will we
use only specify the mandatory parameters and leave the default value for
the other parameters:
GNATCOLL.SQL.Sessions.Setup (Descr => GNATCOLL.SQL.Sqlite.Setup ("library.db"), Max_Sessions => 2);
The first parameter is the same Database_Description we saw earlier (Connecting to the database), but it will be freed automatically by the sessions package, so you should not free it yourself.
Once configure, we can now request a session. Through a session, we can perform queries on the database, make objects persistent, write the changes back to the database,…. We configured the session pool to have at most 2 sessions. The first time we call Get_New_Session, a new session will be created in the pool and marked as busy. While you have a reference to it in your code (generally as a local variable), the session belongs to this part of the code. When the session is no longer in scope, it is automatically released to the pool to be reused for the next call to Get_New_Session. If you call Get_New_Session a second time while some part of your code holds a session (for instance in a different task), a new session will be created. But if you do that a third time while the other two are busy, the call to Get_New_Session is blocking until one of the two sessions is released to the pool.
This technics ensures optimal use of the resources: we avoid creating a new session every time (with the performance cost of connecting to the database), but also avoid creating an unlimited number of sessions which could saturate the server. Since the sessions are created lazily the first time they are needed, you can also configure the package with a large number of sessions with a limited cost.
Let’s then take a new session in our code:
Session : constant Session_Type := Get_New_Session;
and let’s immediately write our first simple query. A customer comes at the library, handles his card and we see his id (1). We need to look up in the database to find out who he is. Fortunately, there is no SQL to write for this:
C : ORM.Detached_Customer'Class := Get_Customer (Session, Id => 1);
The call to Get_Customer performs a SQL query transparently, using prepared statements for maximum efficiency. This results in a Customer object.
ORM is the package that was generated automatically by gnatcoll_db2ada. For each table in the database, it generates a number of types:
This type represents a row of the Customers table. It comes with a number of primitive operations, in particular one for each of the fields in the table. Such an object is returned by a cursor, similarly to what was described in the previous section (Writing your own cursors). This object is no longer valid as soon as the cursor moves to the next row (in the currently implementation, the object will describe the next row, but it is best not to rely on this). As a benefit, this object is light weight and does not make a copy of the value of the fields, only reference the memory that is already allocated for the cursor.
This object redefines the equality operator (“=”) to compare the primary key fields to get expected results.
A detached object is very similar to the Customer object, but it will remain valid even if the cursor moves or is destroyed. In fact, the object has made a copy of the value for all of its fields. This object is heavier than a Customer, but sometimes easier to manager. If you want to store an object in a data structure, you must always store a detached object.
A detached object also embeds a cache for its foreign keys. In the context of our demo for instance, a Book object was borrowed by a customer. When returning from a query, the book knows the id of that customer. But if call B.Borrowed_By this returns a Detached_Customer object which is cached (the first time, a query is made to the DBMS to find the customer given his id, but the second time this value is already cached).
One cache create a Detached_Customer from a Customer by calling the Detach primitive operation.
This type extends a Forward_Cursor (Getting results). In addition to the usual Has_Row and Next operations, it also provides an Element operation that returns a Customer for easy manipulation of the results.
This type extends a Direct_Cursor. It also adds a Element operation that returns a Customer element.
This type is the base type to perform queries on the DBMS. A manager provides a number of primitive operations which end up creating a SQL query operation in the background, without making that explicit.
Let’s first write a query that returns all books in the database:
declare M : Books_Managers := All_Books; BL : Book_List := M.Get (Session); B : Book; begin while BL.Has_Row loop B := BL.Element; Put_Line ("Book: " & B.Title); Put_Line (" Borrowed by: " & B.Borrowed_By.Last); BL.Next; end loop; end;
The manager M corresponds to a query that returns all the books in the database. The second line then executes the query on the database, and returns a list of books. We then traverse the list. Note how we access the book’s title by calling a function, rather than by the index of a field as we did with GNATCOLL.SQL.Exec with Value(B, 0). The code is much less fragile this way.
The line that calls Borrowed_By will execute an additional SQL query for each book. This might be inefficient if there is a large number of books. We will show later how this can be optimized.
The manager however has a lot more primitive operations that can be used to alter the result. Each of these primitive operations returns a modified copy of the manager, so that you can easily chain calls to those primitive operations. Those operations are all declared in the package GNATCOLL.SQL.ORM.Impl if you want to look at the documentation. Here are those operations:
Get and Get_Direct
As seen in the example above, these are the two functions that execute the query on the database, and returns a list of objects (respectively a Customer_List and a Direct_Customer_List).
Returns a copy of the manager that does not return twice a row with the same data (in SQL, this is the “DISTINCT” operator)
Limit (Count : Natural; From : Natural := 0)
Returns a copy of the manager that returns a subset of the results, for instance the first Count ones.
Order_By (By : SQL_Field_List)
Returns a copy of the manager that sorts the results according to a criteria. The criteria is a list of field as was defined in GNATCOLL.SQL. We can for instance returns the list of books sorted by title, and only the first 5 books, by replacing M with the following:
M : Books_Managers := All_Books.Limit (5).Order_By (Books.Title);
Returns a subset of the result matching a criteria. There are currently two versions of Filter: one is specialized for the table, and has one parameter for each field in the table. We can for instance return all the books by Alexandre Dumas by using:
M : Books_Managers := All_Books.Filter (Author => "Dumas");
This version only provides the equality operator for the fields of the table itself. If for instance we wanted all books with less than 50 pages, we would use the second version of filter. This version takes a GNATCOLL.SQL.SQL_Criteria similar to what was explained in previous sections, and we would write:
M : Books_Managers := All_Books.Filter (Condition => Books.Pages < 50);
More complex conditions are possible, involving other tables. Currently, the ORM does not have a very user-friendly interface for those, but you can always do this by falling back partially to SQL. For instance, if we want to retrieve all the books borrowed by user “Smith”, we need to involve the Customers table, and thus make a join with the Books table. In the future, we intend to make this join automatic, but for now you will need to write:
M : Books_Managers := All_Books.Filter (Books.FK (Customers) and Customers.Last = "Smith"); -- SQL query: SELECT books.pages, books.borrowed_by, books.id, -- books.title, books.author, books.published -- FROM books, customers -- WHERE books.borrowed_by=customers.id AND customers.last='Smith'
This is still simpler code than we were writing with GNATCOLL.SQL because we do not have to specify the fields or tables, and the results are objects rather than fields with specific indexes.
Select_Related (Depth : Integer; Follow_Left_Join : Boolean)
This function returns a new manager that will retrieve all related objects. In the example we gave above, we mentioned that every time B.Borrowed_By was called, this resulted in a call to the DBMS. We can optimize this by making sure the manager will retrieve that information. As a result, there will be a single query rather than lots. Be careful however, since the query will return more data, so it might sometimes be more efficient to perform multiple smaller queries.
Depth indicates on how many levels the objects should be retrieved. For instance, assume we change the schema such that a Book references a Customer which references an Address. If we pass 1 for Depth, the data for the book and the customer will be retrieved. If however you then call B.Borrowed_By.Address this will result in a query. So if you pass 2 for Depth the data for book, customers and addresses will be retrieved.
The second parameter related to efficiency. When a foreign key was mentioned as NOT NULL in the schema, we know it is always pointing to an existing object in another table. Select_Related will always retrieve such objects. If, however, the foreign key can be null, i.e. there isn’t necessarily a corresponding object in the other table, the SQL query needs to use a LEFT JOIN, which is less efficient. By default, GNATColl will not retrieve such fields unless Follow_Left_Join was set to True.
In our example, a book is not necessarily borrowed by a customer, so we need to follow the left joins:
M : Books_Managers := All_Books.Filter (Books.FK (Customers) and Customers.Last = "Smith") .Select_Related (1, Follow_Left_Join => True); -- SQL query: SELECT books.pages, books.borrowed_by, books.id, -- books.title, books.author, books.published, -- customers.id, customers.first, customers.last -- FROM (books LEFT JOIN customers ON books.borrowed_by=customers.id) -- WHERE books.borrowed_by=customers.id AND customers.last='Smith'
24.13.1. reverse relationships¶
In fact, the query we wrote above could be written differently. Remember we have already queries the Customer object for id 1 through a call to Get_Customer. Since our schema specified a reverse_name for the foreign key borrowed_by in the table books, we can in fact simply use:
BL := C.Borrowed_Books.Get (Session); -- SQL: SELECT books.pages, books.borrowed_by, books.id, books.title, -- books.author, books.published FROM books -- WHERE books.borrowed_by=1
Borrowed_Books is a function that was generated because there was a reverse_name. It returns a Books_Managers, so we could in fact further filter the list of borrowed books with the same primitive operations we just saw. As you can see, the resulting SQL is optimital.
Let’s optimize further the initial query. We have hard-coded the customer name, but in fact we could be using the same subprograms we were using for prepared statements (Prepared queries), and even prepare the query on the server for maximum efficiency. Since our application is likely to use this query a lot, let’s create a global variable:
M : constant Books_Managers := All_Books.Filter (Books.FK (Customers) and Customers.Id = Integer_Param (1)) .Select_Related (1, Follow_Left_Join => True); MP : constant ORM_Prepared_Statement := M.Prepare (On_Server => True); ... later in the code Smith_Id : constant Natural := 1; BL : Book_List := MP.Get (Session, Params => (1 => Smith_Id));
The last call to Get is very efficient, with timing improvements similar to the ones we discussed on the session about prepared statements (Prepared queries).
24.14. Modifying objects in the ORM¶
The ORM is much more than writing queries. Once the objects are persistent, they can also be simplify modified, and they will be saved in the database transparently.
Let’s start with a simple example. In the previous section, we retrieve an object C representing a customer. Let’s change his name, and make sure the change is in the database:
C := Get_Customer (Session, 1); C.Set_Last ("Smit"); C.Set_First ("Andrew"); Session.Commit;
A reasonable way to modify the database. However, this opens a can of complex issues that need to be dealt with.
When we called Set_Last, this modify the objects in memory. At this point, printing the value of C.Last would indeed print the new value as expected. The object was also marked as modified. But no change was made in the database.
Such a change in the database might in fact be rejected, depending on whether there are constraints on the field. For instance, say there existed a constraint that Last must be the same First (bear with me, this is just an example). If we call Set_Last, the constraint is not satisfied until we also call Set_First. But if the former resulted in an immediate change in the database, it would be rejected and we would not even get a change to call Set_First.
Instead, the session keeps a pointer to all the objects that have been modified. When it is committed, it traverses this list of objects, and commits their changes into the database. In the example we gave above, the call to Commit will thus commit the changes to C in the database. For efficiency, it uses a single SQL statement for that, which also ensures the constraint remains valid:
UPDATE customers SET first='Andrew', last='Smit' WHERE customers.id=1;
We can create a new customer by using similar code:
C := New_Customer; C.Set_First ("John"); C.Set_Last ("Lee"); Session.Persist (C); Session.Commit;
New_Customer allocates a new object in memory. However, this object is not persistent. You can call all the Set_* subprograms, but the object will not be saved in the database until you add it explicitly to a session with a call to Persist, and then Commit the session as usual.
Another issue can occur when objects can be modified in memory. Imagine we retrieve a customer, modify it in memory but do not commit to the database yet because there are other changes we want to do in the same SQL transaction. We then retrieve the list of all customers. Of course, the customer we just modified is part of this list, but the DBMS does not know about the change which currently only exists in memory.
Thankfully, GNATColl takes care of this issue automatically: as we mentioned before, all modified objects are stored in the session. When traversing the list of results, the cursors will check whether the session already contains an element with the same id that it sees in the result, and if yes will return the existing (i.e. modified) element. For instance:
C := Get_Customer (Session, Id => 1); C.Set_Last ("Lee"); CL : Customer_List := All_Customers.Get (Session); while CL.Has_Row loop Put_Line (CL.Element.Last); CL.Next; end loop;
The above example uses CL.Element, which is a light-weight Customer object. Such objects will only see the in-memory changes if you have set Flush_Before_Query to true when you configured the sessions in the call to GNATCOLL.SQL.Sessions.Setup. Otherwise, it will always return what’s really in the database.
If the example was using Detached_Customer object (by calling CL.Element.Detach for instance) then GNATColl looks up in its internal cache and returns the cached element when possible. This is a subtlety, but this is because an Customer only exists as long as its cursor, and therefore cannot be cached in the session. In practice, the Flush_Before_Query should almost always be true and there will be not surprising results.
24.15. Object factories in ORM¶
Often, a database table is used to contain objects that are semantically of a different kind. In this section, we will take a slightly different example from the library. We no longer store the books and the DVDs in separate tables. Instead, we have one single media table which contains the title and the author, as well as a new field kind which is either 0 for a book or 1 for a DVD.
Let’s now look at all the media borrowed by a customer:
C : constant Customer'Class := Get_Customer (Session, Id => 1); ML : Media_List := C.Borrowed_Media.Get (Session); while ML.Has_Row loop case ML.Element.Kind is when 0 => Put_Line ("A book " & ML.Element.Title); when 1 => Put_Line ("A DVD " & ML.Element.Title); end case; ML.Next; end loop;
This code works, but requires a case statement. Now, let’s imagine the check out procedure is different for a book and a DVD (for the latter we need to check that the disk is indeed in the box). We would have two subprograms Checkout_Book and Checkout_DVD and call them from the case. This isn’t object-oriented programming.
Instead, we will declare two new types:
type My_Media is abstract new ORM.Detached_Media with private; procedure Checkout (Self : My_Media) is abstract; type Detached_Book is new My_Media with private; overriding Checkout (Self : Detached_Book); type Detached_DVD is new My_Media with private; overriding Checkout (Self : Detached_DVD);
We could manually declare a new Media_List and override Element so that it returns either of the two types instead of a Media. But then we would also need to override Get so that it returns our new list. This is tedious.
We will instead use an element factory in the session. This is a function that gets a row of a table (in the form of a Customer), and returns the appropriate type to use when the element is detached (by default, the detached type corresponding to a Customer is a Detached_Customer, and that’s what we want to change).
So let’s create such a factory:
function Media_Factory (From : Base_Element'Class; Default : Detached_Element'Class) return Detached_Element'Class is begin if From in Media'Class then case Media (From).Kind is when 0 => return R : Detached_Book do null; end return; when 1 => return R : Detached_DVD do null; end return; when others => return Default; end case; end if; return Default; end Media_Factory; Session.Set_Factory (Media_Factory'Access);
This function is a bit tricky. It is associated with a given session (although we can also register a default factory that will be associated with all sessions by default). For all queries done through this session (and for all tables) it will be called. So we must first check whether we are dealing with a row from the Media table. If not, we simply return the suggested Default value (which has the right Detached_* kind corresponding to the type of From).
If we have a row from the Media table, we then retrieve its kind (through the usual automatically generated function) to return an instance of Detached_Book or Detached_DVD. We use the Ada05 notation for extended return statements, but we could also use a declare block with a local variable and return that variable. The returned value does not need to be further initialized (the session will take care of the rest of the initialization).
We can now write our code as such:
C : constant Customer'Class := Get_Customer (Session, Id => 1); ML : Media_List := C.Borrowed_Media.Get (Session); while ML.Has_Row loop Checkout (ML.Element.Detach); -- Dispatching ML.Next; end loop;
The loop is cleaner. Of course, we still have the case statement, but it now only exists in the factory, no matter how many loops we have or how many primitive operations of the media we want to define.