How to migrate from custom primary key to default id

I have come across this problem myself and ended up writing a reusable (MySQL-specific, though) migration. You can find the code in this repo. I've also written about it in my blog.

As a summary, the steps taken are:

1. Modify your model class like this:

   class Something(models.Model):
       email = models.EmailField(max_length=255, unique=True)
   

2. Add a new migration along these lines:

   app_name = 'app'
   model_name = 'something'
   related_model_name = 'something_else'
   model_table = '%s_%s' % (app_name, model_name)
   pivot_table = '%s_%s_%ss' % (app_name, related_model_name, model_name)
   fk_name, index_name = None, None 
   
   
   class Migration(migrations.Migration):
   
       operations = [
           migrations.AddField(
               model_name=model_name,
               name='id',
               field=models.IntegerField(null=True),
               preserve_default=True,
           ),
           migrations.RunPython(do_most_of_the_surgery),
           migrations.AlterField(
               model_name=model_name,
               name='id',
               field=models.AutoField(
                   verbose_name='ID', serialize=False, auto_created=True,
                   primary_key=True),
               preserve_default=True,
           ),
           migrations.AlterField(
               model_name=model_name,
               name='email',
               field=models.EmailField(max_length=255, unique=True),
               preserve_default=True,
           ),
           migrations.RunPython(do_the_final_lifting),
       ]
   

   where

   def do_most_of_the_surgery(apps, schema_editor):
       models = {}
       Model = apps.get_model(app_name, model_name)
   
       # Generate values for the new id column
       for i, o in enumerate(Model.objects.all()):
           o.id = i + 1
           o.save()
           models[o.email] = o.id
   
       # Work on the pivot table before going on
       drop_constraints_and_indices_in_pivot_table()
   
       # Drop current pk index and create the new one
       cursor.execute(
           "ALTER TABLE %s DROP PRIMARY KEY" % model_table
       )
       cursor.execute(
           "ALTER TABLE %s ADD PRIMARY KEY (id)" % model_table
       )
   
       # Rename the fk column in the pivot table
       cursor.execute(
           "ALTER TABLE %s "
           "CHANGE %s_id %s_id_old %s NOT NULL" %
           (pivot_table, model_name, model_name, 'VARCHAR(255)'))
       # ... and create a new one for the new id
       cursor.execute(
           "ALTER TABLE %s ADD COLUMN %s_id INT(11)" %
           (pivot_table, model_name))
   
       # Fill in the new column in the pivot table
       cursor.execute("SELECT id, %s_id_old FROM %s" % (model_name, pivot_table))
       for row in cursor:
           id, key = row[0], row[1]
           model_id = models[key]
   
           inner_cursor = connection.cursor()
           inner_cursor.execute(
               "UPDATE %s SET %s_id=%d WHERE id=%d" %
               (pivot_table, model_name, model_id, id))
   
       # Drop the old (renamed) column in pivot table, no longer needed
       cursor.execute(
           "ALTER TABLE %s DROP COLUMN %s_id_old" %
           (pivot_table, model_name))
   
   def do_the_final_lifting(apps, schema_editor):
       # Create a new unique index for the old pk column
       index_prefix = '%s_id' % model_table
       new_index_prefix = '%s_email' % model_table
       new_index_name = index_name.replace(index_prefix, new_index_prefix)
   
       cursor.execute(
           "ALTER TABLE %s ADD UNIQUE KEY %s (%s)" %
           (model_table, new_index_name, 'email'))
   
       # Finally, work on the pivot table
       recreate_constraints_and_indices_in_pivot_table()
   

   3. Apply the new migration

This situation is hard to tackle particularly on sqlite which actuall doesn't even have a real ALTER TABLE statement

SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table.

Most of the type, django is doing the changes via a temp table. So you can do that too

Step 1: Create a new model, exactly like

class TempModel(models.Model):
    email = models.EmailField(max_length=255)
    # other fields from your existing model

Note that you don't need to explicitly declare a primary key field. Merely switching it off in the email field is sufficient.

Step 2: make migrations and migrate

Step 3: open your favourite database client and do a:

INSERT INTO myapp_tempmodel(fields,....) SELECT * FROM myapp_oldmodel

Step 4: delete old table, make migrations and migrate

Step 5: rename temp table, make migrations and migrate