Factory Boy set relation value with "model__key"

I'm trying to create some data with Factory Boy but I'm having problems using model__key like in the OrderFactory example of the Factory Boy index page.

This are my factories:

class UserFactory(DjangoModelFactory):
    class Meta:
        model = 'auth.User'
        django_get_or_create = ('username',)

    username = 'user'


class EntityFactory(DjangoModelFactory):
    class Meta:
        model = 'app.Entity'

    user = SubFactory(UserFactory)

And this is what I'm trying run:

EntityFactory(user__username='admin')

But I'm getting:

TypeError: 'user__username' is an invalid keyword argument for this function

Injecting the actual instance works:

admin = UserFactory(username='admin')
EntityFactory(user=admin)

Thanks!