Active Record LEFT OUTER JOIN with support for Polymorphic associations

active_record_extensions.rb

# based on the original Gist from mildmojo
# https://gist.github.com/mildmojo/3724189
#
module ActiveRecord
  module LeftJoinExtension
    extend ActiveSupport::Concern

    module ClassMethods
      # Simple left join taking advantage of existing Rails & Arel code
      def left_joins(*args)
        rel = self.joins(*args).arel
        left_joins = rel.join_sources.map do |join|
          Arel::Nodes::OuterJoin.new(join.left, join.right)
        end
        s = self.joins(left_joins)
        s.bind_values = rel.bind_values
        s
      end

      # Find records of self where no records of given association exist
      def without(assoc_name)
        assoc = reflect_on_association(assoc_name)
        left_joins(assoc_name).where(assoc.table_name => {assoc.klass.primary_key => nil})
      end
    end
  end

  Base.include(LeftJoinExtension)
end

Had a problem with this, if you use it twice.

Company.left_joins(:users).left_joins(:cities)

will use join on users twice. ANy ideas?