Migrating data from "Many-To-Many" to "Many-To-Many Through" in django

Question

I've got a model

class Category(models.Model):
    title           = models.CharField(...)
    entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                             related_name='category_entries',
                                             )

That I wish to refactor to have additional data with each relationship:

class Category(models.Model):
    title           = models.CharField(...)
    entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                             related_name='category_entries',
                                             through='CategoryEntry',
                                             )

But south deletes the existing table. How can I preserve the existing m-t-m relationships?

Answer 1

1. 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'))
   

2. Run the South schema migration.

3. 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.

4. Run the migration.

5. 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()
   

6. 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.