Z3c.sqlalchemy

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- MappedClassBase: new clone() method

- more checks in Model.add()

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- source code polishing

- documentation update

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- sessions were returned from the wrong cache

- moved the rollback/commit handling inside the SessionDataManager
in order to play more nicely with the TPC. See
http://mail.zope.org/pipermail/zodb-dev/2007-May/010996.html

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- createSAWrapper() got a new optional 'name' parameter in order
to register the wrapper automatically instead of using a dedicated
registerSAWrapper(wrapper, name) call

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- added check for the 'mapper_class' attribute (classes from now
on must be a subclass of MapperClassBase)

- a Zope-aware SAWrapper now has a 'connection' property that can
be used to execute SQL statements directly. 'connection' is an
instance of sqlalchemy.Connection and directly tied to the current
Zope transaction.

- changed the caching of the connection and session object for Zope wrapper
since the id of a transaction is not reliable (different transaction
object can re-use the same memory address leading to cache errors)

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- fixed a bug in mapper (unfortunately I forgot to commit a
necessary change)

- removed the 'primary_key' parameter introduced in 0.1.9 because
we don't need. It can be defined within the model using a
PrimaryKeyConstraint()

- createSAWrapper: setting forZope=True for a non-postgres DSN
now also returns a Zope-aware wrapper instance (instead
of a BaseWrapper instance). (Reported by Martin Aspeli)