proxy models: subclass a model without creating a new table

[Ryan Kelly]

Following a discussion with Malcolm on django-users, I've implemented support for what's tentatively named "proxy models". These provide a way for model subclasses to modify the behaviour of a model without creating a new database table. The canonical example seems to be subclassing the User model - you might want to add additional methods/managers/etc, but you still want to pull data directly from the main users table. Under the attached patch this would look like the following:

    class MyUser(User)
        def extra_method(self):
            print "I am a new method"
        class Meta:
            proxy = True

MyUser is now simply a different view onto the main User table, rather than a full-blown subclass with a separate table.

The main body of the patch is to db/models/base.py, where the meta attributes of proxy models are automatically set up. Since it's now possible for a model to be using the same (table,primary key) pair as one of its base classes, I've added a small optimisation in db/models/sql/query.py that avoids a needless join in this case. There's also some tests and a first stab at some documentation.

This patch also integrates with (but doesn't require) ticket #3163, by marking proxy models as unmanaged.

The django-users discussion leading to this patch can be found here: ​http://groups.google.com/group/django-users/browse_thread/thread/fb06ff8e2a296f9c

[Ryan Kelly]

patch implementing proxy models

[Ryan Kelly]

whoops, fixed bad grammar in ticket summary

[Malcolm Tredinnick]

(In [10083]) Fixed #10356 -- Added pure-Python inheritance for models (a.k.a proxy models).

Large portions of this are needed for #5420, so I implemented it fully.
Thanks to Ryan Kelly for an initial patch to get this started.

Refs #5420.