A small ActiveRecord concern to help dry up scopes for belongs_to associations.

scopable.rb

require 'active_support/concern'

module Scopable
  extend ActiveSupport::Concern

  module ClassMethods

    ##
    # Creates scopes for all the belongs_to associations provided as args.
    # For example 'scope_for :user' creates the following scopes.
    #
    # def self.for_user(user)
    #   where(:user_id => user.id)
    # end
    #
    # def self.for_user_id(user_id)
    #   where(:user_id => user_id)
    # end
    def scope_for(*association_syms)
      association_syms.each do |association_sym|
        association = self.reflect_on_association(association_sym)

        define_singleton_method("for_#{association.name}") do |object|
          where(association.foreign_key.to_sym => object.id)
        end

        define_singleton_method("for_#{association.name}_id") do |object_id|
          where(association.foreign_key.to_sym => object_id)
        end
      end
    end

  end

end