In Django/South HOWTO create an instance of a model from a different app during DataMigration

问题

I need to perform a datamigration of a model Answer in app Question. In that script there is a dependency such that I need to create an instance of a model Chapter which is in the app Journal. So, I coded it as follows:

def forwards(self, orm):
    for answer_object in orm.Answer.objects.all():

        #This Works.
        blog, is_created = orm['blog.Post'].objects.get_or_create(title=answer_object.answer[:100])
        blog.save()

        #This DOES NOT work
        chapter, is_created = orm['journal.Chapter'].objects.get_or_create(content_object=blog)
        chapter.save()
        #cleanup task, not relevant to this question below
        answer_object.chapter_ptr = chapter
        answer_object.save()

But as expected this throws an error on " orm['journal.Chapter'].objects.get_or_create(content_object=blog)" saying that

django.core.exceptions.FieldError: Cannot resolve keyword 'content_object' into field.

This is presumably due to content_object being a GenericForeignKey so some operations are not allowed. But I also tried other alternatives for creating the "chapter" object like,

chapter = orm['journal.Chapter'](content_object=blog)
ERROR > TypeError: 'content_object' is an invalid keyword argument for this function

and

chapter = orm.journal.Chapter(content_object=blog)
 ERROR > AttributeError: The model 'journal' from the app 'questions' is not available in this migration. (Did you use orm.ModelName, not orm['app.ModelName']?)

So where am I going wrong? Any pointers appreciated. Thanks.

UPDATE

So since my earlier approach was failing I tried a new tack. The model whose instantiation was failing in my code above i.e. Chapter in the Journal app, I decided to create a datamigration for that instead. I also made sure to --freeze the models I am referring to in the forwards definition. Now this should have been straight forward, I would think. I have my forward code as follows -

def forwards(self, orm):

    for answer_object in orm['questions.Answer'].objects.all():

        #Works, AGAIN!
        blog, is_created = orm['blog.Post'].objects.get_or_create(title=answer_object.answer[:100])
        blog.save()

        # DOES NOT WORK, AGAIN!
        chapter = orm.Chapter(rank=1, content_object=blog)       
        chapter.save()

I would have thought that now since I am creating instance of a model (Chapter) which exists in the subject app (Journal) everything should have worked out. But i am getting the same error.

TypeError: 'content_object' is an invalid keyword argument for this function

It fails at the same point, namely, "content_object". I will post below the model definition if that might help.

class Chapter(models.Model):

    rank = models.IntegerField()

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey()

UPDATE 2 Wanted to add that all the models being touched in these forwards methods, namely - blog, chapter, questions; are fully defined in the 00n_*.py files created by South's schemamigration.

回答1:

After getting help from Rob and folks on the South & Django user groups I was able to resolve this issue. Below is the definition of my forwards datamigration script.

def forwards(self, orm):

    for answer_object in orm['questions.Answer'].objects.all():


        blog, is_created = orm['blog.Post'].objects.get_or_create(title=answer_object.answer[:100])
        blog.save()

        #I have to manually lookup the content_type ans set it in the chapter creation.
        ct = orm['contenttypes.ContentType'].objects.get(app_label="blog", model="post")    
        chapter = orm.Chapter(rank=1, content_type=ct, object_id=blog.id)

        chapter.save()

回答2:

This has sort of been answered before here Django South: Creating schemamigration for more than one app

Basically south doesn't currently support multiple applications.

If you don't want to entangle your applications even more than they are, I would use raw SQL in a db.execute as a quick fix.

It looks like blog and journal are very inter-related. Are you sure you want them in separate applications?

回答3:

In order to create a migration via manage.py, pass --freeze=other_app as arguments to add the models definitions of this other_app to the migration itself.

回答4:

This is a comment on Chantz's answer, which worked great for me (Don't have the rep to post this as a comment)

To save some others some time, you also need to freeze the contenttype app when you create your migration:

python manage.py datamigration yourapp migrationname --freeze contenttypes

来源：https://stackoverflow.com/questions/6976879/in-django-south-howto-create-an-instance-of-a-model-from-a-different-app-during