How to change ancestor of an NDB record?
The only way to change the ancestor of an entity is to delete the old one and create a new one with a new key. This must be done for all child (and grand child, etc) entities in the ancestor path. If this isn't possible, then your listed solution works.
This is required because the ancestor path of an entity is part of its unique key. Parents of entities (i.e., entities in the ancestor path) need not exist, so changing a parent's key will leave the children in the datastore with no parent.
In the High-Replication Datastore (I'm using NDB), the consistency is eventual. In order to get a guaranteed complete set, ancestor queries can be used. Ancestor queries also provide a great way to get all the "children" of a particular ancestor with kindless queries. In short, being able to leverage the ancestor model is hugely useful in GAE.
The problem I seem to have is rather simplistic. Let's say I have a contact record and a message record. A given contact record is being treated as the ancestor for each message. However, it is possible that two contacts are created for the same person (user error, different data points, whatever). This situation produces two contact records, which have messages related to them.
I need to be able to "merge" the two records, and bring put all the messages into one big pile. Ideally, I'd be able to modify ancestor for one of the record's children.
The only way I can think of doing this, is to create a mapping and make my app check to see if record has been merged. If it has, look at the mappings to find one or more related records, and perform queries against those. This seems hugely inefficient. Is there more of "by the book" way of handling this use case?