Question

I have two relationships between models in Django, a many-to-many between Foo and Bar and a foreign key on Foo pointed towards Bar.

When I do a query that involves both Foo and Bar, django insists on using the Foreign Key instead of the M2M to do the join.

(The M2M is the real data here, the Foreign Key is just a bit of caching so I can get the most recent Bar created by a certain method.)

So for example (where foos is the many-to-many relationship name on Bar)

Bar.objects.filter(foos__attribute = True)

Doesn’t return all of the Bars with that attribute, but only the one Bar that Foo is pointed at with the FK. How can I force it to use the M2M? Or is this a bad idea completely?

Answer 1

I figured it out, and it was definitely an instance of ‘should have used real code for the example’.

I had the many-to-many relation on the Bar table called ‘foos’ (lets say). The foreign key was automatically creating a related_name on the Bar table called ‘foo’. I was not actually calling:

Bar.objects.filter(foos__attribute = True)

Like I said I was. I was calling:

Bar.objects.filter(foo__attribute = True)

Which was using the automatically-created related name of the Foreign Key ‘foo’ instead of the name of the many-to-many table ‘foos’.

So lessons learned:

• Don’t let django ever decide the related name for you
• Be careful with pluralization!
• Post real examples on SA