get returns multiple objects when model includes a ManyToManyField in the default ordering

[max@…]

Django throws a MultipleObjectsReturned error when getting a single instance of a model that includes a ManyToManyField in the default ordering.

It seems that the related table is joined with an inner join and so the resulting query has extra rows.
I traced the join to the function get_ordering of django\db\models\sql\query.py.

This simple example illustrates the issue best:

from django.db import models

class Topping(models.Model):
    name  = models.CharField(max_length = 50)

    def __unicode__(self):
        return self.name


class Pizza(models.Model):
    name = models.CharField(max_length = 50)
    toppings = models.ManyToManyField(Topping)

    def __unicode__(self):
        return self.name

    class Meta:


# test with the following 
# create some toppings
tomato = Topping(name="Tomato")
tomato.save()
mozzarella = Topping(name="Mozzarella")
mozzarella.save()

# create a pizza
pizza = Pizza(name="Margareta")
pizza.save()

# add the toppings
pizza.toppings.add(tomato)
pizza.toppings.add(mozzarella)

# select the pizza
p = Pizza.objects.get(pk=1)

# You should get the following error:
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "c:\python25\lib\site-packages\django\db\models\manager.py", line 82, in get return self.get_query_set().get(*args, **kwargs)
  File "c:\python25\lib\site-packages\django\db\models\query.py", line 285, in get % (self.model._meta.object_name, num, kwargs))
MultipleObjectsReturned: get() returned more than one Pizza -- it returned 2! Lookup parameters were {'pk': 1}

[max@…]

Example models and test code

[Malcolm Tredinnick]

"Doctor it hurts when I do this".. "Then don't do that, sir".

As noted in the documentation for ordering, it is legal to include multi-valued fields because there are some situations where it returns sensible results, so prohibiting them always leads to undesired restrictions. That does not mean that including them is a sensible thing to do and you should always never do it.

This isn't a bug in Django because the functionality is needed in some cases. The solution is not to include that field in your default ordering (ordering by multiple valued results hardly ever makes sense).