How to traverse a GenericForeignKey in Django?

问题:

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') [<Foo: Foo object>]  # 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.

回答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')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')) 

文章来源: How to traverse a GenericForeignKey in Django?