Extra fields on many-to-many relationships

[jedlau@…]

Existing text

As you are using an intermediate model, you can also query on its attributes:

# Find all the members of the Beatles that joined after 1 Jan 1961
>>> Person.objects.filter(
...     group__name='The Beatles',
...     membership__date_joined__gt=date(1961,1,1))
[<Person: Ringo Starr]

Bug Description
I'm surprised that this query worked, since Person doesn't have a group attribute. I think the group attribute is supposed to be reached through the intermediate model, membership:

# Find all the members of the Beatles that joined after 1 Jan 1961
>>> Person.objects.filter(
...     membership__group__name='The Beatles',
...     membership__date_joined__gt=date(1961,1,1))
[<Person: Ringo Starr]

[Aymeric Augustin]

This works as designed, but you have to look at it the other way round :)

Start with a regular many-to-many between Person and Group. You can write Person.objects.filter(group='xxx'). Technically, the database does two JOINS (because of the intermediate table) but that doesn't appear in the Python API -- I can't imagine something that would make sense in Person.objects.filter(???_group='xxx').

Now, you want to save some attributes on the relation, and you add an intermediate model. You still expect Person.objects.filter(group='xxx') to work like it used to! However, if you want to query the attributes of the intermediate model, Django needs a way to identify it, hence the membership__ prefix.

In other words: with an intermediate model, you have two possible prefixes:

• membership__ => queries the intermediate model
• group__ => queries the target model

I hopes this helps!