Make models hashable

Before adding this, please consider adding the app and model name into the hash as well as the PK. Otherwise the hashes of Article(pk=1) and Author(pk=1) are identical.

I don't think hash collisions for different models have a big performance penalty (if any). You usually end up having the same type of objects in a set/list (populated from MyModel?.objects.all() or similar). The pk gives a good hash data in this case.

Replying to Bastian Kleineidam <calvin@debian.org>:

I don't think hash collisions for different models have a big performance penalty (if any). You usually end up having the same type of objects in a set/list (populated from MyModel?.objects.all() or similar). The pk gives a good hash data in this case.

That's not entirely true when dealing with the content types framework.

I implemented a hash function independent of having seen this ticket and arrived at basically the same logic Bastian's using. The common case will be objects of the same type, using the pk as a hash is very fast (quite a bit faster than hashing a tuple of class and pk) and satisfies Python's requirements, and even if you do get the odd collision in the case of mixed content types, it doesn't matter that much. Simplicity wins here.

(In [7132]) Fixed #2936, #6500 -- Added a hash() method to Models (since we implement our own eq method).