Design and implementation

Overview

The database backend is split into multiple parts, each having a backend specific implementation

• common/database/Database<backend>.py - Implements common/database/DatabaseInterface.py which is a wrapper for access to the database, including querying. See DatabaseInterface for further details.
• common/database/DB<backend>.py - Implements common/database/DBProxy.py which is a high level interface to functionality needed to make Python objects persistent. See DBProxy for further details.
• common/database/DB<backend>Select.py - Implements common/database/DBSelect.py which is a high level interface to functionality search for persistent objects using a Python-like query syntax. See dbselect for further details.
• ...

The following example demonstrates the simplest possible method to query a default database backend, in this case Oracle

from common.database.Database import database

database.connect()
for row in database.execute_select('SELECT :1 FROM DUAL', 42):
    print(row)
database.disconnect()

DatabaseInterface

The DatabaseInterface for a specific database backend is imported as Database by common/database/Database.py based on the contents of Env['database_engine']. The DatabaseInterface implementation should also provide a database_api that refers to module that implements the Python DB-API. The reference implementation for the interface is DatabaseOracle in common/database/DatabaseOracle.py.

database_engine	DatabaseInterface	database_api
filebased	DatabaseFilebased	None
MySQL	DatabaseMySQL	MySQLdb
oracle_9i	DatabaseOracle	cx_Oracle
oracle_10g	DatabaseOracle	cx_Oracle
postgresql	DatabasePostgreSQL	psycopg2
sqlite	DatabaseSQLite	sqlite3

DBProxy

The DBProxy class has only a few high level methods that are needed to implement persistency. [1]

• commit(self, other) - store a Python object and all its dependencies in a database backend.
• get_instance(self, cls, *args, **kwargs) - fetch a Pyton instance from a database backend.
• register_class(self, cls) - make a Python class known to a database backend.
• persists(self) - returns whether a Python instance is persistent.

[1]	Ideally, the DBProxy class does not need a database backend specific implementation.

The DBProxy is instantiated by common/database/DBProxyFactory.py based on Env['database_engine'].

database_engine	DBProxy
filebased	DBFilebasedProxy
MySQL	DBMySQLProxy
oracle_9i	DBOracleProxy
oracle_10g	DBOracleProxy
postgresql	DBPostgreSQLProxy
sqlite	DBSQLiteProxy