Django ReverseSingleRelatedObjectDescriptor.__set__ ValueError

[Bradford Wade]

I am creating a custom data migration to automatically create GenericRelation entries in the database, based on existing entries across two different models.

Example models.py:

    

  ...
    class Place
       content_type = models.ForeignKey(ContentType)
       object_id = models.PositiveIntegerField()
       content_object = generic.GenericForeignKey('content_type', 'object_id')




    class Restaurant
       name = models.CharField(max_length=60)
       location = models.CharField(max_length=60)




    class House
       location = models.CharField(max_length=60)

Example migration.py:

    

    # -*- coding: utf-8 -*-
    from django.contrib.contenttypes.models import ContentType
    from django.db import models, migrations


    def forwards_func(apps, schema_editor):
        Restaurant = apps.get_model("simpleapp", "Restaurant")
        House = apps.get_model("simpleapp", "House")
        Location = apps.get_model("simpleapp", "Location")


        db_alias = schema_editor.connection.alias


        content_type = ContentType.objects.using(db_alias).get(
            app_label="simpleapp",
            model="restaurant"
        )


        for restaurant in Restaurant.objects.using(db_alias).all():
            Place.objects.using(db_alias).create(
                content_type=content_type,
                object_id=restaurant.id)
       
        content_type = ContentType.objects.using(db_alias).get(
            app_label="simpleapp",
            model="house"
        )
        for house in House.objects.using(db_alias).all():
            Place.objects.using(db_alias).create(
                content_type=content_type,
                object_id=house.id)


       
    class Migration(migrations.Migration):


        dependencies = [
            ('simpleapp', '0010_location')
        ]


        operations = [
            migrations.RunPython(
                forwards_func,
            ),
        ]

When I run this (Django 1.7.4) I get   

    File ".../django/db/models/fields/related.py", line 597, in __set__
    self.field.rel.to._meta.object_name,
    ValueError: Cannot assign "<ContentType: restaurant>": "Place.content_type" must be a "ContentType" instance.

If I comment out the stanza raising the value error the the actual Django model it works as expected. 

Should this exception be raised in the first place? Is this a bug in Django or am I doing it wrong?

[Simon Charette]

Hi Bradford,

You must retrieve the ContentType model from the `apps` instead of importing it just like the other models you're using in your forwards function.

ContentType = apps.get_model("contenttypes", "ContentType")


The reason why you're getting such a strange exception is because you're trying to assign a content type instance from a foreign `apps` instance.

I guess the error message could be improved when the expected model apps doesn't match the assigned one, feel free to open a ticket with this enhancement proposal.

Simon