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
Le mercredi 18 mars 2015 17:07:40 UTC-4, Bradford Wade a écrit :
>
> 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?
>
>
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