Cannot create an instance of a model with GenericForeignKey in migration

Question

IMPORTANT: This question is no longer relevant.

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In a Django 1.7 migration I try to create Comment entries programatically with the following code:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals
from django.db import models, migrations
class Migration(migrations.Migration):
 def create_genericcomment_from_bookingcomment(apps, schema_editor):
 BookingComment = apps.get_model('booking', 'BookingComment')
 Comment = apps.get_model('django_comments', 'Comment')
 for comment in BookingComment.objects.all():
 new = Comment(content_object=comment.booking)
 new.save()
 dependencies = [
 ('comments', '0001_initial'),
 ('django_comments', '__first__'),
 ]
 operations = [
 migrations.RunPython(create_genericcomment_from_bookingcomment),
 ]

And it produces an error: TypeError: 'content_object' is an invalid keyword argument for this function

However, the same code (i.e. Comment(content_object=comment.booking)) works when executed in the shell.

I tried to create a blank model with new = Comment() and then set all the necessary fields manually but even though I set content_type and object_pk fields accordingly, they content_type was not actually saved and I received django.db.utils.IntegrityError: null value in column "content_type_id" violates not-null constraint

Any idea how to properly create a model with a generic foreign key in a migration? Or any workaround?

• tutuca: Can you paste the models? At least the relevant bit? I'm running into the same situation trying to create a simple model that is the target of a M2M field. The model itself has no relation field.

Answer 1

This is an issue of migrations' model loader. You load your models using default

Comment = apps.get_model('django_comments', 'Comment')

It loads the Comment model in some special way, so some features like generic relations don't work.

There is a bit hacky solution: load your models as usual:

from django_comments import Comment

• rgammans: Unfortunately this isn't even a solution. It works until you add a field to Comment; at that point during migration the upto date model generates SQL for the schema version which is to be applied in a later migration. So any long term project which has older database hanging around can no longer apply ir migrations