[1.6.x] Fixed #21431 -- GenRel->FK list_filter regression in admin

def get_model_from_relation(field):

    if isinstance(field, models.related.RelatedObject):

    if hasattr(field, 'get_path_info'):

        return field.get_path_info()[-1].to_opts.model

    elif isinstance(field, models.related.RelatedObject):

        return field.model

    elif getattr(field, 'rel'): # or isinstance?

        return field.rel.to

from django.conf import settings, global_settings

from django.core import mail

from django.core.exceptions import ImproperlyConfigured

from django.core.files import temp as tempfile

from django.core.urlresolvers import reverse

# Register auth models with the admin.

from django.contrib.admin.models import LogEntry, DELETION

from django.contrib.admin.sites import LOGIN_FORM_KEY

from django.contrib.admin.util import quote

from django.contrib.admin.validation import ModelAdminValidator

from django.contrib.admin.views.main import IS_POPUP_VAR

from django.contrib.admin.tests import AdminSeleniumWebDriverTestCase

from django.contrib.auth import REDIRECT_FIELD_NAME

class NamespacedAdminKeepChangeListFiltersTests(AdminKeepChangeListFiltersTests):

    admin_site = site2

class AdminGenericRelationTests(TestCase):

    def test_generic_relation_fk_list_filter(self):

        """

        Validates a model with a generic relation to a model with

        a foreign key can specify the generic+fk relationship

        path as a list_filter. See trac #21428.

        """

        class GenericFKAdmin(admin.ModelAdmin):

            list_filter = ('tags__content_type',)

        validator = ModelAdminValidator()

        try:

            validator.validate_list_filter(GenericFKAdmin, Plot)

        except ImproperlyConfigured:

            self.fail("Couldn't validate a GenericRelation -> FK path in ModelAdmin.list_filter")