Question

I am writing an application in Google App Engine with python and I want to sort users and user posts into groups. Users will be able to tag a post with a group ID and then that post will be displayed on the group page.

I would also like to relate the users to the groups so that only members of a group can tag a post with that group ID and so that I can display all the users of a group on the side. I am wondering if it would be more efficient to have a property on the user which will have all of the groups listed (I am thinking max 10 or so) or would it be better to have a property on the Group model which lists all of the users (possibly a few hundred).

Is there much of a difference here?

Answer 1

Your data model should derive from the most likely use cases. What are you going to retrieve?

A. Show a list of groups to a user.
B. Show a list of users in a group.

Solution:

If only A, store unindexed list of groups in a property of a user entity.
If both, same as above but indexed.
If only B, store unindexed list of users in a property of a group entity.

NB: If you make a property indexed, you cannot put hundreds of user ids in it – it will lead to an exploding index.