How to create API methods in Google App Engine that have several decedents/ancestors

Question

I am having trouble understanding how to structure an ancestor tree with several decedents. Suppose I have a model like this (every Entity has a Long id):

Answer 1

I ran into the same exact problem, and have currently implemented something similar to @feroult, though I am still looking for a better method. Let me explain some of the logic behind this decision.

Let's assume we read a list of some resource that is at least a few levels down in the tree. The "comment" example above could be one example. In this case, if we were to use datastore keys on the client side, then yes, our API would be very RESTful in that the key itself would encompass the path to the root entity. The problem however is that this method is inefficient since the ancestor path is repeated in every key.

The alternative is to create a tree like structure on the client that resembles the tree like data from the datastore. If we want all the comments made by a user, then we only need to have the user id reside in memory once. There is no reason to have it represented in a key for every comment. Which then leads us to the ugly mess of sending the full ancestor path /users/{userId}/posts/{postId}/comments/{commentId} in the RESTful api.

At the end of the day, no matter how the actually path looks, the same information must be passed to the datastore. This is critical. Google Datastore cannot find a comment without the ancestor path.

This means that the problem can be reduced to one of semantics. Perhaps the compromise is to create an equivalent client side key factory that can generate a key from the ancestor ids and then send it to the server. In this case, repetition would be avoided AND the RESTful API would still be clean.

Would be interested in hearing other thoughts.