Transactions
************


Transaction
===========

For each committed transaction SQLAlchemy-Continuum creates a new
Transaction record.

Transaction can be queried just like any other sqlalchemy declarative
model.

   from sqlalchemy_continuum import transaction_class
   Transaction = transaction_class(Article)

   # find all transactions
   session.query(Transaction).all()


UnitOfWork
==========

For each database connection SQLAlchemy-Continuum creates an internal
UnitOfWork object. Normally these objects are created at before flush
phase of session workflow. However you can also force create unit of
work before this phase.

   uow = versioning_manager.unit_of_work(session)

Transaction objects are normally created automatically at before flush
phase. If you need access to transaction object before the flush phase
begins you can do so by calling the create_transaction method of the
UnitOfWork class.

   transaction = uow.create_transaction(session)

The version objects are normally created during the after flush phase
but you can also force create those at any time by calling
make_versions method.

   uow.make_versions(session)


Workflow internals
==================

Consider the following code snippet where we create a new article.

   article = Article()
   article.name = u'Some article'
   article.content = u'Some content'
   session.add(article)
   session.commit()

This would execute the following SQL queries (on PostgreSQL)

1. INSERT INTO article (name, content) VALUES (?, ?)
      params: ('Some article', 'Some content')

2. INSERT INTO transaction (issued_at) VALUES (?)
      params: (datetime.utcnow())

3. INSERT INTO article_version (id, name, content, transaction_id)
   VALUES (?, ?, ?, ?)
      params: (<article id from query 1>, 'Some article', 'Some
      content', <transaction id from query 2>)
