On Nov 13, 2013, at 7:00 AM, Mark Bird <mark.a.b...@gmail.com> wrote:
> I am trying to write a generic auditing extension, but I have some
> inconsistencies I'd like to iron out if possible.
>
> The problem I have is that when an object is updated via a relationship
> rather than a Column attribute, the value returned from
> attributes.get_history is a SQLAlchemy object, and not the value that is in
> the database. This is more pronounced on a many-to-many relationship - the
> secondary table isn't marked as new/dirty, even though that is the table that
> will actually have data inserted into it. Instead the 2 tables joined by the
> secondary table are marked as dirty, although no data is changed in those
> tables. I understand this makes sense at the SQLAlchemy level - but is there
> a way to get the actual database changes?
yeah I’ve tried to stress in the documentation, using “secondary” in
relationship() in conjunction with an ORM object that is actually mapped to
that table will lead to confusion. There is no history present for the table
that you specify for “secondary”, it is only subject to inserts/deletes based
on changes to the collection in which it refers to, e.g. Foo.bars, and these
inserts/deletes are calculated and executed within a closed flush action. The
FooBar class does not play a role of any kind in this operation since you have
not instantiated a FooBar object.
You need to make a choice if you want to work with Foo->FooBar->Bar objects, or
if you want to work with Foo->Bar directly with “secondary” in between, and
then stick to just one or the other.
>
> A code example to illustrate this:
>
> from sqlalchemy.types import Integer
> from sqlalchemy import Column, ForeignKey, create_engine
> from sqlalchemy.orm import relationship, backref, attributes, sessionmaker
> from sqlalchemy.ext.declarative import declarative_base
> from sqlalchemy.orm.interfaces import SessionExtension
>
> BASE = declarative_base()
>
> ENGINE = create_engine("sqlite://")
>
> class AuditListener(SessionExtension):
> def after_flush(self, session, *args):
> for attr in ("new", "dirty", "deleted"):
> print attr
> for obj in getattr(session, attr):
> print " %s" % obj.__class__.__name__
> for col in obj.__mapper__.iterate_properties:
> added, unchanged, deleted = attributes.get_history(obj,
> col.key)
> if added:
> print " %s: %s" % (col.key, str(added))
>
> SESSION = sessionmaker(
> bind=ENGINE,
> extension=(AuditListener())
> )
>
> class FooBar(BASE):
> __tablename__ = 'foobar'
> foo_id = Column('foo_id',
> ForeignKey('foo.foo_id'),
> primary_key=True)
>
> bar_id = Column('bar_id',
> ForeignKey('bar.bar_id'),
> primary_key=True)
>
> class Foo(BASE):
> __tablename__ = 'foo'
> foo_id = Column('foo_id',
> Integer,
> primary_key=True)
>
> bars = relationship('Bar',
> secondary=FooBar.__table__,
> backref=backref('foos'))
>
> class Bar(BASE):
> __tablename__ = 'bar'
> bar_id = Column('bar_id',
> Integer,
> primary_key=True)
>
>
> def example():
> BASE.metadata.create_all(ENGINE, checkfirst=False)
>
> session = SESSION()
> f = Foo(foo_id=1)
> b1 = Bar(bar_id=1)
> b2 = Bar(bar_id=2)
> session.add(f)
> session.add(b1)
> session.add(b2)
> session.commit()
>
> # add relationship between f and b1 & b2:
> f.bars = [b1, b2]
> session.commit()
>
> if __name__ == '__main__':
> example()
>
> If you run this code you will see this output:
>
> new
> Bar
> bar_id: [2]
> Foo
> foo_id: [1]
> Bar
> bar_id: [1]
> dirty
> deleted
> new
> dirty
> Bar
> Foo
> bars: [<__main__.Bar object at 0x27b4910>, <__main__.Bar object at
> 0x277b050>]
> Bar
> deleted
>
> As you can see, the output when the many-to-many relationship is committed is
> to denote an update to the bars relationship of Foo. There is nothing
> indicating there are 2 new FooBar entries created.
>
> Any ideas?
>
> Thanks,
>
> Mark.
>
>
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