Migrating data from "Many-To-Many" to "Many-To-Many Through" in django
Create your intermediate model without any extra fields, for now. Give it a unique constraint to match the existing one and specify the table name to match the existing one:
class CategoryEntry(models.Model): category = models.ForeignKey(Category) entry = models.ForeignKey(Entry) class Meta: db_table='main_category_entries' #change main_ to your application unique_together = (('category', 'entry'))
Run the South schema migration.
Edit the generated schema migration script and comment-out all the forwards and backwards entries, since you'll be re-using the existing intersection table. Add
pass
to complete the methods.Run the migration.
Update any existing code. As it says in https://docs.djangoproject.com/en/dev/topics/db/models/#many-to-many-relationships, "Unlike normal many-to-many fields, you can't use add, create, or assignment to create relationships" so you'll need to modify any existing application code, e.g.
c.entry.add(e)
could become:
try: categoryentry = c.categoryentry_set.get(entry = e) except CategoryEntry.DoesNotExist: categoryentry = CategoryEntry(category=c, entry=e) categoryentry.save()
and:
e.category_entries.add(c)
could become:
categoryentry = CategoryEntry(category=c, entry=e) #set extra fields here categoryentry.save()
and:
c.entry.remove(e)
could become:
categoryentry = c.categoryentry_set.get(entry = e) categoryentry.delete()
Once this initial pseudo migration has been done, you should then be able to add the extra fields to the
CategoryEntry
and create further migrations as normal.
I'd do it in the following way:
Add the
CategoryEntry
class to the model, and do an auto schema migration. This will add an empty table containing the properties ofCategoryEntry
. To be noted, the older M2M table remains untouched sincethrough='CategoryEntry'
has not yet been added.Do a data migration to copy all data from the existing M2M table to the table created in step 1. To do so, run the
datamigration
command, and edit methodsforward()
andbackward()
in the auto generated migration script accordingly.Now add
through='CategoryEntry'
part (just the way you wanted), and do a schemamigration. this will drop the old M2M table.
In Django 1.7+ built-in migrations, the way the "code state" (i.e. the code definition of models) is calculated is different, and requires a different solution.
In South (Django pre-1.7), the entire "code state" is saved in each migration — but in Django 1.7+ built-in migrations, it's derived from looking at the whole set of migrations, so you need to specify the "code state" change in a migration without altering the database.
Like above, this will need to be done in a few steps.
Create an intermediate model like in the answer above:
class CategoryEntry(models.Model): category = models.ForeignKey(Category, on_delete=models.CASCADE) entry = models.ForeignKey(Entry, on_delete=models.CASCADE) class Meta: db_table = 'main_category_entries' #change main_ to your application unique_together = ('category', 'entry')
Create an auto-migration with
django-admin.py makemigrations
and modify the code; move the operations list into thestate_operations
argument of amigrations.SeparateDatabaseAndState
operation, and leave thedatabase_operations
list empty. It should look like:class Migration(migrations.Migration): operations = [ migrations.SeparateDatabaseAndState( state_operations=[ migrations.CreateModel(CategoryEntry..) ... ], database_operations=[] ), ]
Edit the
CategoryEntry
to contain what you want and create a new auto-migration withdjango-admin.py makemigrations