Drop many-to-many relationships to be only many-to

App Engine is really hard to use for many to many relationships. It does
not allow you to use the default JPA annotations but forces you to store
keys only. And doing batch updates is also impossible because you can
touch at most 5 entities in a single transaction (and a transaction
automatically maps to the lifetime of an entitymanager).

It's the best not to use many to many relations in App Engine (at least
on datanucleus v2) and only store a single side of the relationship.