implementing __hash__ in mapped classes

[Victor Olex]

What is a good practice for implementing __hash__ method in mapped
classes? Left unimplemented an object id is returned. Within the same
session SQLAlchemy ensures that the same record is mapped to one
instance of the mapped class. However if object is not in session
(expunged or session closed) and the same record gets retrieved from
another session its identities will be different. It would make sense
to return hash of primary key if object is persisted in database.

Are there any gotchas in doing the following (assume id field is whole
primary key):

def __hash__(self):
return id(self) if self.id is None else hash(self.id)

Can you also comment in context of

[Michael Bayer]

SQLalchemy goes through a lot of effort to ensure that its own identity map and other related functions are using object identity as referenced by the python "is" operator as well as the id() function. You are free to implement any kind of __hash__() method, as well as a custom __nonzero__(), and the operation of the ORM will remain unaffected.

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