South trying to change Piece.barcode ?

gistfile1.txt

# -*- coding: utf-8 -*-
import datetime
from south.db import db
from south.v2 import SchemaMigration
from django.db import models


class Migration(SchemaMigration):

    def forwards(self, orm):
        # Adding field 'Piece.unsortable'
        db.add_column('warehouse_pieces_piece', 'unsortable',
                      self.gf('django.db.models.fields.BooleanField')(default=False),
                      keep_default=False)

        # Adding field 'Piece.barcode_absent'
        db.add_column('warehouse_pieces_piece', 'barcode_absent',
                      self.gf('django.db.models.fields.BooleanField')(default=False),
                      keep_default=False)


        # Changing field 'Piece.barcode'
        db.alter_column('warehouse_pieces_piece', 'barcode_id', self.gf('warehouse_pieces.models.ForeignKeySaveForm')(null=True, on_delete=models.SET_NULL, to=orm['barcodes.Barcode']))

    def backwards(self, orm):
        # Deleting field 'Piece.unsortable'
        db.delete_column('warehouse_pieces_piece', 'unsortable')

        # Deleting field 'Piece.barcode_absent'
        db.delete_column('warehouse_pieces_piece', 'barcode_absent')


        # Changing field 'Piece.barcode'
        db.alter_column('warehouse_pieces_piece', 'barcode_id', self.gf('django.db.models.fields.related.ForeignKey')(null=True, on_delete=models.SET_NULL, to=orm['barcodes.Barcode']))