Rails idiom to avoid duplicates in has_many :through

Question

I have a standard many-to-many relationship between users and roles in my Rails app:

class User < ActiveRecord::Base
  has_many :user_roles
  has_many :ro         


        

Answer 1

You can use a combination of validates_uniqueness_of and overriding << in the main model, though this will also catch any other validation errors in the join model.

validates_uniqueness_of :user_id, :scope => [:role_id]

class User
  has_many :roles, :through => :user_roles do
    def <<(*items)
      super(items) rescue ActiveRecord::RecordInvalid
    end
  end
end