heres the source to session.connection()
def connection(self, mapper, **kwargs):
"""returns a Connection corresponding to the given mapper.
used by the execute()
method which performs select operations for Mapper and Query.
if this Session is transactional,
the connection will be in the context of this session's
transaction. otherwise, the connection
is returned by the contextual_connect method, which some
Engines override to return a thread-local
connection, and will have close_with_result set to True.
the given **kwargs will be sent to the engine's
contextual_connect() method, if no transaction is in progress."""
if self.transaction is not None:
return self.transaction.connection(mapper)
else:
return self.get_bind(mapper).contextual_connect(**kwargs)
so session.connection() is going to behave differently if you have
opened a SessionTransaction or not, since it wants to be part of the
existing context but only if there is one.
also, session.create_transaction() doesnt use any connections. the
connection is only acquired when the session first tries to get a
connection off of the SessionTransaction, at which point
SessionTransaction uses contextual_connect() and begins a transaction
on it, and then stores that connection within the scope of
SessionTransaction.
in your test #6, calling sessiontransaction.rollback() does nothing
because the sessiontransaction was never used to get a connection.
if youre really looking to understand the session, its worth a read of
session.py, which is a pretty transparent module and should be pretty
straightforward to understand.
as far as threadlocal impacting contextual_connection, absolutely. the
docs explain this in great detail at
http://www.sqlalchemy.org/docs/dbengine.myt#dbengine_implicit . in 0.3
I have greatly de-emphasized the "threadlocal" engine strategy since
its really only useful for very specific patterns that most people
don't actually need. without "threadlocal", the contextual_connect and
connect methods of engine do the exact same thing, except
contextual_connect includes the "close_with_result" setting.
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