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

Based on the info provided you have two options:

• Pass full keys around and design your API around that
• decouple your entities ancestor relationships, so that no entity has a parent

Note: after writing this all out, I realized the end API looks the same really either way:

GET /user/{key} - get user info
POST /user/{key}/post/ - create post 
GET /post/{key} - get post
POST /post/{key}/comment/ - create comment
GET /comment/{key} 

Full keys

In this case, {key} is the websafe key.

Advantages:

• you can maintain transactional and consistency control
• allows you to change parent entity relationship later without migrating old data and breaking old links (in my experience this is a massive win)
• Allows you to mix kinds (e.g. Have a Post kind and a Post2 kind, or an ImagePost kind - useful for polymorphism or for breaking migrations)

Decouple entities as ancestors

In this case {key} is the id, and there is no entity hierarchy

Dis/Advantages:

• Simplicity
• You need to infer the kind based on the URL
• listing posts for a user or comments for a post would always be eventual
• no migration path if you need to introduce transaction groups

Overall benefits of structuring your API like this:

• internal coupling for consistency and transactionality not exposed through API
• aligns with restful API as resource concept
• supports either model

In my experience you should absolutely do the first option, you will probably get your data model wrong a few times, and being able to change/migrate it is an absolute win.