Query lookup types should be scoped to the last joined field's model

[Adrian Holovaty]

This is kind of obscure and best described by example. I have two models:

from django.db import models
from django.contrib.gis.db import models as geo_models

# Note this is a geo-aware model.
class Place(geo_models.Model):
    geom = geo_models.GeometryField()

# Note this is NOT a geo-aware model.
class Person(models.Model):
    name = models.CharField(max_length=50)
    hometown = models.ForeignKey(Place)

I'd like to be able to do this:

Person.objects.filter(hometown__geom__intersects=some_geometry)

...but that doesn't work. It gives me this error:

django.core.exceptions.FieldError: Join on field 'geom' not permitted. Did you misspell 'intersects' for the lookup type?

This happens because Person isn't a geo-aware model, so it doesn't know that __intersects is a valid lookup type. I can fix it by changing Person to extend django.contrib.gis.db.models and adding the GeoManager, but that feels hacky because the Person model itself doesn't have any geo fields.

The solution could be to change Django's join code so that it looks at the model of the last field in the chain when determining whether a lookup is valid. I haven't looked into how difficult this would be, though. I think it's just enough of an edge case that it wouldn't be worth fixing if it required a ton of refactoring and hoop-jumping. But if it's easy, let's do it.