Duplicate items when ordering by Generic Relation in Django

27 January 2010

When the optional ordering = … attribute of a model’s Meta class contains a GenericRelation from the Content Types framework in Django, there is no way to eliminate duplicate items being returned, even when using .distinct() (since .order_by(…) is applied by the ORM only after the SQL SELECT DISTINCT clause is built).

This problem is current as of Django 1.3 and has no generic solution as far as I know.

This problem will manifest when two or more different Models instances are attached to instances of the Model with the Generic Foreign Key. In the following example, we have Model X which has a Generic Foreign Key, and two Models, Y and Z, which have Generic Relations to X and ordering based on these relations.

from django.db.models import CharField

from django.contrib.contenttypes.models import ContentType

from django.contrib.contenttypes.generic import GenericForeignKey, GenericRelation
class X(Model):
    """ This Model has a unique name and is attached to another Model through a
        Generic relation.
    """

    # The unique name of this X.
    name = CharField(u'name', unique = True, max_length = 128, null = False)

    # The Content Type of the generic relation to the attached content.
    content_type = models.ForeignKey(ContentType, null = True, blank = True,
        related_name = 'x_contents')

    # The ID of the attached content.
    content_id = models.PositiveIntegerField(null = True, blank = True)

    # The attached content.
    content = GenericForeignKey('content_type', 'content_id')

    def __unicode__(self):
        """ The textual representation of this Model is its name.
        """

        return self.name

    class Meta:

        app_label = 'cmf'

        # Order by X's name.
        ordering = ['name', ]
class Y(Model):
    """ This Model acts as content for an X, and has a convenient reverse
        Generic Relation to X.
    """

    # The unique name of this Y.
    name = CharField(u'name', unique = True, max_length = 128, null = False)

    # The X this Y is attached to.
    xs = GenericRelation(X, content_type_field = 'content_type',
        object_id_field = 'content_id')

    def __unicode__(self):
        """ The textual representation of this Model is its name.
        """

        return self.name

    class Meta:

        # Order by X's name.
        ordering = ['xs__name', ]
class Z(Model):
    """ This Model acts as content for an X, and has a convenient reverse
        Generic Relation to X.
    """

    # The unique name of this Z.
    name = CharField(u'name', unique = True, max_length = 128, null = False)

    # The X this Z is attached to.
    xs = GenericRelation(X, content_type_field = 'content_type',
        object_id_field = 'content_id')

    def __unicode__(self):
        """ The textual representation of this Model is its name.
        """

        return self.name

    class Meta:

        app_label = 'cmf'

        # Order by X's name.
        ordering = ['xs__name', ]

The following sequence of statements demonstrates this problem:

x1 = X.objects.create(name = 'first x')
x2 = X.objects.create(name = 'second x')
x3 = X.objects.create(name = 'third x')
x4 = X.objects.create(name = 'fourth x')
y1 = Y.objects.create(name = 'first y')
y2 = Y.objects.create(name = 'second y')
z1 = Z.objects.create(name = 'first z')
z2 = Z.objects.create(name = 'second z')
x1.content = y1
x1.save()
x2.content = z1
x2.save()
x3.content = y2
x3.save()
x4.content = z2
x4.save()
X.objects.all()
[<X: first x>, <X: fourth x>, <X: second x>, <X: third x>]
Y.objects.all()
[<Y: first y>, <Y: first y>, <Y: second y>]

Notice that ‘first y’ appears twice. The SQL COUNT statement will not have any ordering applied, and so returns the correct value:

Y.objects.count()
2

However, when ordering is applied, the duplicate instance is present:

len(Y.objects.all())
3

Tags: , , ,
Categories: Django