REST - Implementing HABTM in the real world

Hi again - another REST question!

I think I’ve got a good grasp on it all now, except for how to
implement HABTM properly. I understand you need turn the join model
into a controller, such as ‘memberships’ below, and then use CRUD on
that to perform joining actions.

class Group < ActiveRecord::Base

has_many :memberships
has_many :users, :through => :memberships

class User < ActiveRecord::Base

has_many :memberships
has_many :groups, :through => :memberships

class Membership < ActiveRecord::Base

belongs_to :user
belongs_to :group

What I don’t understand is how that actually works in your code. Let’s
say I’m looking at all of the groups (GET /groups), and want to get
the current user to join one. Presumably, I’d POST to /memberships?

But how would I handle if that failed, or they didn’t have permission
to do so? And how would I handle the controller returning to the page
it came from? Because I could hit the same action from both sides of
the HABTM relationship. (User joining a group, and group selecting a

Are there any examples of this out there?

Thanks a lot!