Proper way to handle removing a member from a group in REST - web-services

Question

I'm designing a REST service which organizes groups and users.
For example:
GET /groups - gets all the groups
GET /groups/1 - gets a specific group
GET /groups/1/users - gets the users in the group
GET /users/1 - the actual user, which may be in multiple groups
POST /groups/1/users - with the post parameters of user_id=1 to add a user to a group
What would be the appropriate way to handle this?
DELETE /groups/1/users/1 seems to be a valid way to do it but then the GET to the same url would return the user record which is a duplicate of the resource /user/1?
or should it be DELETE /groups/1/users?user_id=1?
Wondering which is the most RESTful way to do this.

Answer 1

I think that a good design would make explicit the membership of a user within a particular group - as a separate resource. So, there are groups, users, and the membership of a user within a group.
Therefore, GET /groups/1/users would return a list of membership resource identifiers: /groups/1/users/{member_id} on which you could do a DELETE. Each of these "memberships" is of course associated with a particular user, so you would have to somehow know which member_id is associated with which user_id. The easiest way to do this is to make member_id have the same semantics as user_id, as you suggest in the question (so /groups/1/users/1 means "user 1's membership in group 1"). Following that, if you do a GET on /groups/1/users/{member_id} you could just redirect to /users/{user_id}. Or in a more complex example, this resource would no redirect to a user but link to it and also include some other information, for example the date when the user joined the group, her status in the group, etc.
The other option I can think of would utilize the PATCH method to modify the collection resources (/groups/1/users): see https://www.rfc-editor.org/rfc/rfc5023. But using a DELETE seems more natural.