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Minimal Zope/SQLAlchemy transaction integration

Project description

Introduction

The aim of this package is to unify the plethora of existing packages integrating SQLAlchemy with Zope’s transaction management. As such it seeks only to provide a data manager and makes no attempt to define a zopeish way to configure engines.

You need to understand SQLAlchemy for this package and this README to make any sense. See http://sqlalchemy.org/docs/.

Running the tests

This package is distributed as a buildout. Using your desired python run:

$ python bootstrap.py

This will download the dependent packages and setup the test script, which may be run with:

$ ./bin/test

To enable testing with your own database set the TEST_DSN environment variable to your sqlalchemy database dsn. Two-phase commit behaviour may be tested by setting the TEST_TWOPHASE variable to a non empty string. e.g:

$ TEST_DSN=postgres://test:test@localhost/test TEST_TWOPHASE=True bin/test

Example

This example is lifted directly from the SQLAlchemy declarative documentation. First the necessary imports.

>>> from sqlalchemy import *
>>> from sqlalchemy.ext.declarative import declarative_base
>>> from sqlalchemy.orm import scoped_session, sessionmaker, relation
>>> from zope.sqlalchemy import ZopeTransactionExtension
>>> import transaction

Now to define the mapper classes.

>>> Base = declarative_base()
>>> class User(Base):
...     __tablename__ = 'test_users'
...     id = Column('id', Integer, primary_key=True)
...     name = Column('name', String(50))
...     addresses = relation("Address", backref="user")
>>> class Address(Base):
...     __tablename__ = 'test_addresses'
...     id = Column('id', Integer, primary_key=True)
...     email = Column('email', String(50))
...     user_id = Column('user_id', Integer, ForeignKey('test_users.id'))

Create an engine and setup the tables. Note that for this example to work a recent version of sqlite/pysqlite is required. 3.4.0 seems to be sufficient.

>>> engine = create_engine(TEST_DSN, convert_unicode=True)
>>> Base.metadata.create_all(engine)

Now to create the session itself. As zope is a threaded web server we must use scoped sessions. Zope and SQLAlchemy sessions are tied together by using the ZopeTransactionExtension from this package.

>>> if SA_0_5:
...     Session = scoped_session(sessionmaker(bind=engine,
...     twophase=TEST_TWOPHASE, extension=ZopeTransactionExtension()))

The exact arguments depend on the version. Under SQLAlchemy 0.4 we must also supply transactional=True (equivalent to autocommit=False, which is default under 0.5).

>>> if SA_0_4:
...     Session = scoped_session(sessionmaker(bind=engine,
...     twophase=TEST_TWOPHASE, extension=ZopeTransactionExtension(),
...     transactional=True, autoflush=True,))

Call the scoped session factory to retrieve a session. You may call this as many times as you like within a transaction and you will always retrieve the same session. At present there are no users in the database.

>>> session = Session()
>>> session.query(User).all()
[]

We can now create a new user and commit the changes using Zope’s transaction machinary, just as Zope’s publisher would.

>>> session.add(User(name='bob'))
>>> transaction.commit()

Engine level connections are outside the scope of the transaction integration.

>>> engine.connect().execute('SELECT * FROM test_users').fetchall()
[(1, ...'bob')]

A new transaction requires a new session. Let’s add an address.

>>> session = Session()
>>> bob = session.query(User).all()[0]
>>> bob.name
u'bob'
>>> bob.addresses
[]
>>> bob.addresses.append(Address(email='bob@bob.bob'))
>>> transaction.commit()
>>> session = Session()
>>> bob = session.query(User).all()[0]
>>> bob.addresses
[<Address object at ...>]
>>> bob.addresses[0].email
u'bob@bob.bob'
>>> bob.addresses[0].email = 'wrong@wrong'

To rollback a transaction, use transaction.abort().

>>> transaction.abort()
>>> session = Session()
>>> bob = session.query(User).all()[0]
>>> bob.addresses[0].email
u'bob@bob.bob'
>>> transaction.abort()

By default, zope.sqlalchemy puts sessions in an ‘active’ state when they are first used. ORM write operations automatically move the session into a ‘changed’ state. This avoids unnecessary database commits. Sometimes it is necessary to interact with the database directly through SQL. It is not possible to guess whether such an operation is a read or a write. Therefore we must manually mark the session as changed when manual SQL statements write to the DB.

>>> session = Session()
>>> conn = session.connection()
>>> users = Base.metadata.tables['test_users']
>>> conn.execute(users.update(users.c.name=='bob'), name='ben')
<sqlalchemy.engine.base.ResultProxy object at ...>
>>> from zope.sqlalchemy import mark_changed
>>> mark_changed(session)
>>> transaction.commit()
>>> session = Session()
>>> session.query(User).all()[0].name
u'ben'
>>> transaction.abort()

If this is a problem you may tell the extension to place the session in the ‘changed’ state initially.

>>> Session.configure(extension=ZopeTransactionExtension('changed'))
>>> Session.remove()
>>> session = Session()
>>> conn = session.connection()
>>> conn.execute(users.update(users.c.name=='ben'), name='bob')
<sqlalchemy.engine.base.ResultProxy object at ...>
>>> transaction.commit()
>>> session = Session()
>>> session.query(User).all()[0].name
u'bob'
>>> transaction.abort()

Changes

0.4 (2009-01-20)

Bugs fixed:

  • Only raise errors in tpc_abort if we have committed.
  • Remove the session id from the SESSION_STATE just before we de-reference the session (i.e. all work is already successfuly completed). This fixes cases where the transaction commit failed but SESSION_STATE was already cleared. In those cases, the transaction was wedeged as abort would always error. This happened on PostgreSQL where invalid SQL was used and the error caught.
  • Call session.flush() unconditionally in tpc_begin.
  • Change error message on session.commit() to be friendlier to non zope users.

Feature changes:

  • Support for bulk update and delete with SQLAlchemy 0.5.1

0.3 (2008-07-29)

Bugs fixed:

  • New objects added to a session did not cause a transaction join, so were not committed at the end of the transaction unless the database was accessed. SQLAlchemy 0.4.7 or 0.5beta3 now required.

Feature changes:

  • For correctness and consistency with ZODB, renamed the function ‘invalidate’ to ‘mark_changed’ and the status ‘invalidated’ to ‘changed’.

0.2 (2008-06-28)

Feature changes:

  • Updated to support SQLAlchemy 0.5. (0.4.6 is still supported).

0.1 (2008-05-15)

  • Initial public release.

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