Question

I have an existing Django project that has several models using concrete inheritance of a base class. After closer consideration, and after reading about what people like Jacob Kaplan-Moss have to say about it, using this concrete inheritance is unnecessary in my case. I would like to migrate to using an abstract base class instead.

The thing that makes this complicated is that my site is live and I have user entered data. Thus, I’ll need to keep all my data intact throughout this transition.

I’ll give an example to be more concrete:

Before:

app1/models.py:

class Model1(base_app.models.BaseModel):
    field1 = models.CharField(max_length=1000)
    field2 = models.CharField(max_length=1000)

app2/models.py:

class Model2(base_app.models.BaseModel):
    field1 = models.CharField(max_length=1000)
    field2 = models.CharField(max_length=1000)

base_app/models.py:

class BaseModel(models.Model):
    user = models.ForeignKey(User)
    another_field = models.CharField(max_length=1000)

After:

app1/models.py:

class Model1(base_app.models.BaseModel):
    field1 = models.CharField(max_length=1000)
    field2 = models.CharField(max_length=1000)

app2/models.py:

class Model2(base_app.models.BaseModel):
    field1 = models.CharField(max_length=1000)
    field2 = models.CharField(max_length=1000)

base_app/models.py:

class BaseModel(models.Model):
    user = models.ForeignKey(User)
    another_field = models.CharField(max_length=1000)

    class Meta:
        abstract = True

Right now, my plan is to first add the abstract = True to the BaseModel. Then,for each model that uses BaseModel, one at a time:

• Use south to migrate the database and create this migration using the –auto flag
• Use a south data migration. For instance, I would loop through each object in Model1 to fetch the object in BaseModel that has the same pk and copy the values for each field of the BaseModel object to the Model1 object.

So first, will this work? And second, is there a better way to do this?

Update:

My final solution is described in detail here:

http://www.markliu.me/2011/aug/23/migrating-a-django-postgres-db-from-concrete-inhe/

Answer 1

1. Add NewBaseModel, we use different name so it doesn’t conflict with current non-abstract one (South would actually delete BaseModel otherwise).

   class NewBaseModel(models.Model):
       user = models.ForeignKey(User)
       another_field = models.CharField(max_length=1000)
   
       class Meta:
           abstract = True
   

2. Set Model1 and Model2 to inherit from NewBaseModel

3. Run schemamigration –auto, 2 new fields will be added to Model1 and Model2
4. Run datamigration –empty and fill new fields from values in BaseModel
5. Load production db and double check everything migrated correctly
6. Remove BaseModel and rename NewBaseModel to BaseModel
7. Run schemamigration –auto (this should work 😉 )
8. Deploy!

NOTE: Use orm variable when migrating to use current state of your model schema.