HBTM relationship validation question

gistfile1.rb

# Basically I am building a multi-domain/tenant app and in some cases I need to specify what users are valid on what domains. A dumbed down contrived example would be:

class User < ActiveRecord::Base
  has_and_belongs_to_many :domains
end

class Domain < ActiveRecord::Base
  has_and_belongs_to_many :users
end

u = User.first

# this autosaves the relationship and no validators/callbacks on User or Domain are invoked.
u.domains << Domain.first

# For the domains relationship on the user I want to validate that it is never empty, and that the current domain the user is accessing is never removed. There is also a possibility that I would never want any previously added domain to be removed.