Multi-table inheritance breaks GenericRelation querying

[robwa]

Minimal example to reproduce. Create two models:

class Generic(models.Model):                                                                   
    content_type = models.ForeignKey('contenttypes.ContentType')                               
    object_id = models.PositiveIntegerField()                                                  
    obj = GenericForeignKey()                                                                  
                                                                                               
class Vehicle(models.Model):                                                                   
    associations = GenericRelation('Generic', related_query_name='vehicles')

Create a Vehicle and a Generic object pointing to the Vehicle. The following query returns the expected result:

In [2]: Generic.objects.filter(vehicles__id=1)
Out[2]: <QuerySet [<Generic: Generic object>]>

Add a third model. Don't change any data. Now the query returns an empty query set:

class Bike(Vehicle):                                                                          
    pass

In [2]: Generic.objects.filter(vehicles__id=1)
Out[2]: <QuerySet []>

[robwa]

I'm not sure if this is helpful, but I found

In [44]: Generic._meta.get_field('vehicles').field.model
Out[44]: foo.models.Bike

[robwa]

Now I can see why this happens:

• GenericRelation fields are created as private fields.
• Private fields are explicitly inherited by child classes (​db.models.base).
• When constructing the relation tree, private fields are included (​db.models.options).
• When constructing the fields map, relations to private fields of child classes simply overwrite the relations to the parents (​db.models.options).

Where is the point to fix this?

[robwa]

Maybe the reverse relationship should be removed from GenericRelations when copying them to child classes.

[Paulo]

I think the behavior should be consistent with how a normal ForeignKey field behaves in multi-table inheritance.
In the following scenario:

class Car(models.Model):
    owner = models.ForeignKey(Owner, related_name='cars')
    some_id = models.CharField(max_length=100)

class Car2(Car):
    pass

Running

Owner.objects.filter(cars__isnull=True)

Will output:

SELECT "paulotest_owner"."id",
       "paulotest_owner"."nick"
FROM "paulotest_owner"
LEFT OUTER JOIN "paulotest_car" ON ("paulotest_owner"."id" = "paulotest_car"."owner_id")
WHERE "paulotest_car"."id" IS NULL

So we should prevent the reverse relationship from being set.

@Robert, if you would like to work on this, I can un-assign it, I'm planning to tackle it within the next two weeks.

[robwa]

@Paulo, I will try to work on this. If I come up with a patch, I'd like to discuss it with you, as I'm not very familiar with the django model internals.

[robwa]

Added a ​pull request.

[robwa]

Updated the ​PR.

[Carlton Gibson]

Updated changes look good.

[Tim Graham <timograham@…>]

In 4ab027b:

Fixed #28988 -- Fixed queries when a GenericRelation is used with multi-table inheritance.