Trac SQL Database API

[b]Trac SQL Database API[/b]

Trac uses a very thin layer on top of the standard Python Database API 2.0 for interfacing with supported relational database systems such as SQLite or PostgreSQL.

Note that Trac prior to version 0.9 used the PySQLite APIs directly, and has no support for other database systems. This document describes the thin DB API abstraction layer introduced in version 0.9.

You can find the specifics of the database API in the trac.db package. This package provides:
Simple pooling of database connections
Iterable cursors
Selection of DB modules based of connection URIs.

[b]Accessing the Database[/b]

Code accessing the database in Trac go through this layer simply by using the Environment method get_db_cnx() in order to get a pooled database connection. A database cursor can be obtained from the pooled database connection, and the code uses the cursor in conjunction with SQL statements to implement the behavior.

from trac.env import Environment
def myFunc():
    env = Environment('/path/to/projenv')
    db = env.get_db_cnx()
    cursor = db.cursor()
    # Execute some SQL statements
    db.commit()
    return

Note that you should always make sure that db won't get garbage collected while cursor is still used, as the collection will do a rollback and close the cursors (avoid in particular doing cursor = env.get_db_cnx().cursor(), see r8878).

The get_db_cnx method looks at the value of the database configuration option in trac.ini, which should contain a database connection URI. The default value for this option tells Trac to use an SQLite database inside the environment directory:

[trac]
database = sqlite:db/trac.db

The connection URI syntax has been designed to be compatible with that provided by SQLObject (see also the Wiki page on SQLObject connections). The only supported URI schemes at this point are sqlite and postgres.

[b]Pooled Connections[/b]

The Environment method get_db_cnx() returns a connection from the pool of connections. This connection needs to be returned, and Trac is written so that the return will happen automatically by the garbage collector if the code is written to follow a simple rule. When the garbage collector determines the pooled database connection is no longer being used, it's del method will return the pooled connection to the pool for reuse. If you have set a lexical variable in the function's body to the pooled connection, this typically occurs when the function is returning. In the example above of myFunc it occurs at the return statement since db is a variable local to myFunc

[b]Rules for DB API Usage[/b]

Different DB API modules have different ways to pass parameters to the cursors' execute method, and different ways to access query results. To keep the database API as thin as possible, the Trac team has decided to use a relatively common subset in all database code.

[b]Parameter passing[/b]

Always use the "format" parameter style, and always with %s (because that's the only type that pyPgSQL supports). Statement parameters always need to be passed into execute as an actual sequence (list or tuple).

So the following statements are okay:

cursor.execute("SELECT author,ipnr,comment FROM wiki WHERE name=%s", [thename])
cursor.execute("SELECT id FROM ticket WHERE time>=%s AND time<=%s", (start, stop))

The following uses are not okay:

cursor.execute("SELECT author,ipnr,comment FROM wiki WHERE name=?", thename)
cursor.execute("SELECT id FROM ticket WHERE time>=%i AND time<=%i", start, stop)

At any cost, try avoiding the use of string formatting to get values into the SQL statement. The database automatically escapes values you pass using execute() arguments, the same is not true if you use string formatting, opening your code up to SQL injection attacks.

On the other hand, you must use string formatting to dynamically specify names of tables or columns, i.e. anything that is not a value as such:

cursor.execute("SELECT time FROM %s WHERE name=%%s" % table, (thename,))

[b]Retrieving results[/b]

For convenience, cursors returned by the database connection object are iterable after having executed an SQL query. Individual fields in result rows may only be accessed using integer indices:

cursor.execute("SELECT author,ipnr,comment FROM wiki WHERE name=%s", (thename,))
for row in cursor:
    print 'Author: %s (%s)' % (row[0], row[1])
    print row[2]

Accessing fields using the column names is not supported by all database modules, so it should not be used. Automatically unpacking rows into tuples of named variables often provides better readability:

cursor.execute("SELECT author,ipnr,comment FROM wiki WHERE name=%s", (thename,))
for author, ipnr, comment in cursor:
    print 'Author: %s (%s)' % (author, ipnr)
    print comment

[b]Guidelines for SQL Statements[/b]

As you may know, support for SQL varies among different database systems. The requirements of Trac are relatively trivial, so we try to stick to a common subset that is supported by the majority of databases.

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