Symfony2 Form Rest Api only add to Relation - rest

Question

I am using Symfony2 as Rest Api for a JS Frontend App. I came across a scenario where I want users to "invite" (=add) Users to a Group. But I want to only allow them to add Users to the existing Relation and not "overwrite" the whole relation, which is the standard behaviour in combination with a regular Symfony2 Form.
What would be the best practice to achieve this behaviour?
Additional Comment:
I am using Ember-Data in the frontend and my frontend would probably send a put request with the whole Group including additional users (but not all).
My JSON Payload would look something like this:
{
"usergroup": {
"name":"yxcv2",
"stake":"sdfghj",
"imageName":null,
"userCount":5,
"users":[
5,
6,
7
],
"gameGroup":"13",
}
}
In this scenario User 1,2,3 and 4 are already members of the group. And instead of replacing 1,2,3,4 with 5,6,7, I want to ADD 5,6,7 to the already existing members.

Answer 1

A LINK request should be used to add an item to an existing collection instead of overwriting it with a POST request.
Using a symfony form you'd post the User (id) plus a hidden field _method with value LINK to something like /groups/{id}.
routing would be something like this:
group_invite:
path: /groups/{id}
defaults: { _controller: YourBundle:Group:inviteUser }
methods: [LINK]
You could use FOSRestBundle's implicit resource name definition, too.
For the method override to work the config setting framework.http_method_override needs to be set to true ( = default value - available since symfony version 2.3).
More information can be found in the documentation chapter:
How to use HTTP Methods beyond GET and POST in Routes