title: Method to migrate App Engine models
body: <p>Database migrations are a popular pattern, particularly with Ruby on Rails. Since migrations specify how to mold old data to fit a new schema, they can be helpful when you have production data that must be converted quickly and reliably.</p>
  
  <p>But migrating models in App Engine is difficult since processing all entities sequentially is difficult, and there is no offline operation to migrate everything effectively in one big transaction.</p>
  
  <h3>What are your techniques for modifying a db.Model "schema" and migrating the data to fit the new schema?</h3>