Rails #3847

company.rb

class Company < ActiveRecord::Base
  has_many :relationships
  has_many :users, :through => :relationships, :uniq => true, :dependent => :destroy

  with_options :through => :relationships, :source => :user, :dependent => :destroy,
    :extend => Associations::RelationshipExtension do |co|
      co.has_many :advisors,       :conditions => ["relationships.role = ?", ROLES[:advisor]]
      co.has_many :incubators,     :conditions => ["relationships.role = ?", ROLES[:incubator]]
      co.has_many :past_investors, :conditions => ["relationships.role = ?", ROLES[:past_investor]]
      co.has_many :referrers,      :conditions => ["relationships.role = ?", ROLES[:referrer]]
      co.has_many :team_members,   :conditions => ["relationships.role = ?", ROLES[:team_member]]
  end
end

relationship.rb

class Relationship < ActiveRecord::Base
  belongs_to :user
  belongs_to :company
end

relationship_extension.rb

module Associations
  # Extension methods for Relationship association proxies, such as custom
  # instance-scoped aggregation functions, automatic attribute manipulations
  # when associations are made, etc.
  module RelationshipExtension
    # Add to an association collection, automatically setting role attribute on
    # the join model based on the name of the association.
    #
    # @example Create Relationship with 'Investor' role.
    #   Company.first.investors << User.first
    def <<(*args)
      # only makes sense for Company currently
      if [Company].include? proxy_association.owner.class
        role = proxy_association.reflection.name.to_s.titleize.singularize
        Relationship.send(:with_scope, create: { role: role }) { super }
      else
        super
      end
    end
    alias push <<
    alias concat <<
  end
end

relationship_extension_spec.rb

require 'spec_helper'

describe Associations::RelationshipExtension do
  context '#<<' do
    let(:company) { Company.make! }

    it 'delegates if association base model is not a Company' do
      company.stub(:class) { User }
      expect do
        company.past_investors << User.make
      end.to raise_error(ActiveRecord::RecordInvalid, /Role can't be blank/)
    end
  end
end