Foreign key needs a value from the key's table to match a column in another table.

Answer 1

I'm sure you could do it at the PostgreSQL level with a trigger, which you could add to a Django initial-SQL file so it's automatically created at syncdb.

At the Django model level, in order to get a useful answer you'll have to clarify why you're opposed to overriding the save() method, since that is currently the correct (and perhaps only) way to provide this kind of validation.

Django 1.2 will (hopefully) include a full model validation framework.

Answer 2

You could use the pre_save signal and raise an error if they do no match... The effect would be similar to overridding save (it gets called before save)

The problem is creating/deleting/updating the many-to-many relation will not trigger save (or consequentially pre_save or post_save)

Update

Try using the through argument on your many-to-many relation

That lets you manually define the intermediary table for the m2m relation, which will give you access to the signals, as well as the functions.

Then you can choose signals or overloading as you please