post save

print("instance.data['categories'] before loop",instance.data['categories'])
    for cat in instance.data['categories']:
        print("cat ", cat)
        print("instance.story.categories Before if", instance.story.categories)
        if cat not in instance.story.categories:
            print("instance.story.categories After if", instance.story.categories)
            instance.story.categories.append(cat)
            print("instance.story.categories After append", instance.story.categories)
            story_changed = True
    if story_changed:
        print("story changed ", instance.data['categories'])
        instance.story.save()