Many-many association on the same model (like user_user) doesn't work

The document inside the user_user collection only contains **one user field**. Should it not contain two user fields?

Steps to replicate the issue:
1. Create two users. (POST /user)
2. Add the second user as a friend of the first one. (POST /user/{firstUserId}/user) { childId: secondUserId })
3. Get the friends of the firstUser. (GET /user/{firstUserId}/user)
4. The response object is empty. No friends get returned.

No source file or test is named. Start by reproducing the POST and GET /user/{firstUserId}/user steps, then inspect the user_user collection and the association handling. Done means the relationship stores both users and the GET response returns the added friend.

Written by the indexing model from the issue text.