I gave a presentation a couple years ago at the Twin Cities DevFest conference and I’ve been meaning to post the slides.
The gist of the talk is that with web frameworks like Rails and Django, data migration is a feature of the data model tools. With App Engine Datastore (now Cloud Datastore) you have to do the work yourself. In the talk I give Python examples of how to update the NDB models, how to use deferred tasks and mapper/mapreduce jobs to update existing entities.
The slides are here:
I’m hoping to record myself giving the presentation soon.