Removing an item from a collection in a HATEOAS way - json

Question

I am working on a project whose client uses REST/HATEOAS to get and modify data.
Here is a part of the class diagram: class diagram.
A hal+json HATEOAS response of the request /group/12345 would look like:
{
"id": "12345",
"_links": {
"self": {
"href": "/group/12345"
},
"roles": {
"href": "/group/12345/roles"
}
}
}
To remove a role from this group I could simply execute a DELETE on the specific role. (because every role has to be in exactly one group and moving a role to another group shouldn't be allowed).
So I would add a link with the rel "drop" to the role. Therefore the client knows if or rather when a DELETE request is allowed:
{
"id": "67890",
"_links": {
"self": {
"href": "/roles/67890"
},
"users": {
"href": "/roles/67890/users"
},
"drop": {
"href": "/roles/67890"
}
}
}
So to delete a user the client looks for the drop link. If no link is found, the delete is not allowed. Otherwise it executes a DELETE request on the found link.
But what should I do if I want to remove a user from a role?
I cannot simply delete the user. The role -> user relation is not an aggregation.
How can I tell the client if removing a user from a role is allowed?
To remove a user from role I would use DELETE /groups/12345/roles/67890/users/ABC. And to delete the user I would use DELETE /users/ABC.
So where should I put the "remove user from role" link?
Thank you in advance :)

Answer 1

Links typically have three parts
A context identifier
a link relation type
a target identifier
Frequently, the context identifier is implied by the... umm... context, rather than being made explicit.
From what I can see, hal+json doesn't have a mechanism for explicitly specifying the context identifier, which would mean that you need to rely upon the implicit approach.
That suggests that if you want to be communicating about removing a user from a role, then you need a context that implies the right identifier.
You would normally do this in one of two ways
have a chain of links that leads to a representation of the user in that role, and include the drop role link relation in the list of links
embed a representation of the user in some other representation, and include the drop role link within that embedded resource.
For example, you might have a representation of /roles/67890/users that looks something like
_links: {
self: {
href: /roles/67890/users
}
}
_embedded: {
users: [ {
_links: {
self: /users/ABC
/role/remove : /groups/12345/roles/67890/users/ABC
}
} ,
... ]
}