How to traverse a GenericForeignKey in Django?

Question

I\'m using Django v1.9.4 with PostgreSQL 9.2.14 behind. With the following models:

from django.db import models
from django.contrib.contenttypes.fields impo         


        

Answer 1

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'))