How do I migrate a model out of one django app and into a new one?

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How to migrate using south.

Lets say we got two apps: common and specific:

myproject/
|-- common
|   |-- migrations
|   |   |-- 0001_initial.py
|   |   `-- 0002_create_cat.py
|   `-- models.py
`-- specific
    |-- migrations
    |   |-- 0001_initial.py
    |   `-- 0002_create_dog.py
    `-- models.py

Now we want to move model common.models.cat to specific app (precisely to specific.models.cat).
First make the changes in the source code and then run:

$ python manage.py schemamigration specific create_cat --auto
 + Added model 'specific.cat'
$ python manage.py schemamigration common drop_cat --auto
 - Deleted model 'common.cat'

myproject/
|-- common
|   |-- migrations
|   |   |-- 0001_initial.py
|   |   |-- 0002_create_cat.py
|   |   `-- 0003_drop_cat.py
|   `-- models.py
`-- specific
    |-- migrations
    |   |-- 0001_initial.py
    |   |-- 0002_create_dog.py
    |   `-- 0003_create_cat.py
    `-- models.py

Now we need to edit both migration files:

#0003_create_cat: replace existing forward and backward code
#to use just one sentence:

def forwards(self, orm):
    db.rename_table('common_cat', 'specific_cat') 

    if not db.dry_run:
        # For permissions to work properly after migrating
        orm['contenttypes.contenttype'].objects.filter(
            app_label="common",
            model="cat",
        ).update(app_label="specific")

def backwards(self, orm):
    db.rename_table('specific_cat', 'common_cat')

    if not db.dry_run:
        # For permissions to work properly after migrating
        orm['contenttypes.contenttype'].objects.filter(
            app_label="specific",
            model="cat",
        ).update(app_label="common")

#0003_drop_cat:replace existing forward and backward code
#to use just one sentence; add dependency:

depends_on = (
    ('specific', '0003_create_cat'),
)
def forwards(self, orm):
    pass
def backwards(self, orm):
    pass

Now both apps migrations are aware of the change and life sucks just a little less 🙂
Setting this relationship between migrations is key of success.
Now if you do:

python manage.py migrate common
 > specific: 0003_create_cat
 > common: 0003_drop_cat

will do both migration, and 

python manage.py migrate specific 0002_create_dog
 < common: 0003_drop_cat
 < specific: 0003_create_cat

will migrate things down.

Notice that for upgrading of schema I used common app and for downgrading, I used specific app. That’s because how the dependency here works.

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