Choose your PKs wisely, kids.

gistfile1.py

class Migration(SchemaMigration):

    def forwards(self, orm):
        # OMFG Get ready, we're going to change the primary key!
        # Doing bad things, disabling triggers
        db.execute("ALTER TABLE brainstorm_idea DISABLE TRIGGER ALL;")

        # Dropping foreign key constraint
        db.delete_foreign_key('brainstorm_idea', 'subsite_id')
        db.execute('DROP INDEX brainstorm_idea_subsite_id_like')
        db.delete_index('brainstorm_idea', ['subsite_id'])

        # Removing primary key on 'Subsite', fields ['slug']
        db.delete_primary_key(u'brainstorm_subsite')

        # Adding primary key field 'Subsite.id'
        db.add_column(u'brainstorm_subsite', u'id',
                      self.gf('django.db.models.fields.IntegerField')(blank=True, null=True))

        # WOW. Have to manually create AutoFields. Thanks Django!
        db.execute('CREATE SEQUENCE brainstorm_subsite_id_seq;')
        db.execute("UPDATE brainstorm_subsite SET id = nextval('brainstorm_subsite_id_seq');")
        db.execute("ALTER TABLE brainstorm_subsite ALTER COLUMN id SET DEFAULT nextval('brainstorm_subsite_id_seq');")
        db.execute('ALTER SEQUENCE brainstorm_subsite_id_seq OWNED BY brainstorm_subsite.id;')
        db.execute("SELECT setval('brainstorm_subsite_id_seq', q.i) FROM(SELECT MAX(id) i FROM brainstorm_subsite) q;")

        # Now make it the pk
        db.create_primary_key('brainstorm_subsite', ['id'])

        # Updating foreign key values
        db.execute('''UPDATE brainstorm_idea idea
                      SET subsite_id = subsite.id
                      FROM brainstorm_subsite subsite
                      WHERE(idea.subsite_id = subsite.slug);''')

        # Casting the fk to an integer
        db.execute("ALTER TABLE brainstorm_idea ALTER COLUMN subsite_id TYPE integer USING CAST(subsite_id AS integer);")

        # Re-adding foreign key constraint
        fk_sql = db.foreign_key_sql('brainstorm_idea', 'subsite_id', 'brainstorm_subsite', 'id')
        db.execute(fk_sql)

        # Changing field 'Subsite.slug' to a plain old slugfield
        db.alter_column(u'brainstorm_subsite', 'slug', self.gf('django.db.models.fields.SlugField')(max_length=50))

        # Adding unique constraint on 'Subsite', fields ['slug']
        db.create_unique(u'brainstorm_subsite', ['slug'])

        # Re-enabling triggers
        db.execute("ALTER TABLE brainstorm_idea ENABLE TRIGGER ALL;")