new_contents: # -*- coding: utf-8 -*-
  # Generated by Django 1.11.2 on 2017-07-06 11:34
  from __future__ import unicode_literals
  
  from django.db import migrations
  
  
  def set_blank_to_null(apps, schema_editor):
      for model_name in ('Area', 'Point', 'Level', 'Space', 'LocationGroup'):
          model = apps.get_model('mapdata', model_name)
          model.objects.filter(color='').update(color=None)
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('mapdata', '0014_mapupdate'),
      ]
  
      operations = [
          migrations.RunPython(set_blank_to_null),
      ]
  
subject: set color='' to color=None