has_many :through

gistfile1.rb

class User < ActiveRecord::Base
  attr_accessible :name
  
  has_many :bookings
  has_many :events, :through => :bookings
end

class Event < ActiveRecord::Base
  attr_accessible :title

  has_many :bookings
  has_many :users, :through => :bookings
end


# This is the join model... 
class Booking < ActiveRecord::Base
  belongs_to :user
  belongs_to :event
  attr_accessible :role
end

user = User.create(name: "A User")
event = Event.create(title: "An Event")

user.events << event # works as expected but creates a Booking instance with 'nil' role.

booking = Booking.create(role: "Admin")
user.bookings << booking
event.bookings << booking

# done deal