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 import GenericRelation, GenericForeignKey
from django.contrib.contenttypes.models import ContentType

class Foo(models.Model):
    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    bar = GenericForeignKey('content_type', 'object_id')

class Bar(models.Model):
    foos = GenericRelation(Foo, related_query_name='bars')
    class Meta:
        abstract = True

class BarX(Bar):
    name = models.CharField(max_length=10, default='bar x')

class BarY(Bar):
    name = models.CharField(max_length=10, default='bar y')

Create some instances to demonstrate my problem:

>>> bar_x = BarX.objects.create()
>>> bar_y = BarY.objects.create()
>>> foo1 = Foo.objects.create(bar=bar_x)
>>> foo2 = Foo.objects.create(bar=bar_y)
>>> foo1.bar.name
u'bar x'
>>> foo2.bar.name
u'bar y'

I can't traverse the GFK in django, attempting to filter raises an exception with a message suggesting to add the GenericRelation. But using the generic relation, through related query name bars, is not working reliably. For example:

>>> [foo.bar.name for foo in Foo.objects.all()]
[u'bar x', u'bar y']  # in a pure python loop, it's working
>>> Foo.objects.filter(bar__name='bar x')
FieldError: Field 'bar' does not generate an automatic reverse relation and therefore cannot be used for reverse querying. If it is a GenericForeignKey, consider adding a GenericRelation.
>>> Foo.objects.values_list('bars__name', flat=1)
[None, u'bar y']   # but why None is returned in here?
>>> Foo.objects.filter(bars__name='bar x')
[]  # why no result here?
>>> Foo.objects.filter(bars__name='bar y')
[]  # but this one works?

What am I doing wrong?

Cautionary note to future readers: Templating related_query_name on GenericRelation doesn't work properly on Django 1.9.

Added in Django 1.10 was related_query_name now supports app label and class interpolation using the '%(app_label)s' and '%(class)s' strings, after the fix for #25354 was merged.

If you're on Django 1.10, you can go ahead and put the GenericRelation on the abstract base class and template it like related_query_name='%(app_label)s_%(class)s' to ensure uniqueness across the subclasses.

How to traverse a GenericForeignKey in Django?

1 Answers