How to traverse a GenericForeignKey in Django?

In general, it is not possible to traverse a

GenericForeignKey

in this
direction in the way you are trying. A

GenericForeignKey

can point to any
model in your application at all, not only

Bar

and its subclasses. For that
reason,

Foo.objects.filter(bar__somefield='some value')

cannot know what
target model you have in mind at the moment, and therefore it is impossible to
tell what fields that target model has. In fact, there is no way to pick what
database table to join with when performing such a query – it could be any
table, depending on the value of

Foo.content_type

.

If you do want to use a generic relation in joins, you’ll have to define a

GenericRelation

on the other end of that relationship. That way you can let
Django know which model it is supposed to look for on the other side.

For instance, you can create your

BarX

and

BarY

models like this:

class BarX(Bar):    name = models.CharField(max_length=10, default='bar x')    foos = GenericRelation(Foo, related_query_name='bar_x')class BarY(Bar):    name = models.CharField(max_length=10, default='bar y')    foos = GenericRelation(Foo, related_query_name='bar_y')

If you do this, then you can perform queries like the following:

Foo.objects.filter(bar_x__name='bar x')Foo.objects.filter(bar_y__name='bar y')

However, you have to pick a single target model. That is a limitation that you
cannot really overcome in any way; every database join needs to know in
advance which tables it operates on.

If you absolutely need to allow both

BarX

and

BarY

as the target, you
should be able to list both of them explicitly in your query filter using a

Q

expression:

Foo.objects.filter(Q(bar_x__name='bar x') | Q(bar_y__name='bar y'))