Rails 3 - Extra join conditions for ActiveRecord "through" joins - eg.) table1.account_id = table2.account_id

rails3_activerecord_monkey_patch.rb

module AccountJoinCondition
  def join_with_account(table, constraint)
    s_r = reflection.source_reflection
    # source reflection can be polymorphic
    s_r = reflection if s_r && s_r.options[:polymorphic]
    t_r = reflection.through_reflection
    if s_r && t_r && has_account?(s_r) && has_account?(t_r)
      sr_account_column = account_col(s_r)
      tr_account_column = account_col(t_r)
      constraint = constraint.and(sr_account_column.eq(tr_account_column)) if direct_through_join?(sr_account_column, tr_account_column, constraint)
    end
    join_without_account(table, constraint)
  end
  def self.included(receiver)
    receiver.send :include, CustomJoinMethods
    receiver.alias_method_chain :join, :account
  end
  module CustomJoinMethods
    private
      def has_account?(reflection_obj)
        reflection_obj.klass.column_names.include?("account_id")
      end
      def account_col(reflection_obj)
        reflection_obj.klass.arel_table["account_id"]
      end
      # for joins made directly on a class and a through association using active record query syntax, 
      # source_reflection's table_name is different in the first join
      def direct_through_join?(sr_account_column, tr_account_column, constraint)
        sr_account_column.relation.name.eql?(constraint.left.relation.name) &&  tr_account_column.relation.name.eql?(constraint.right.relation.name)
      end
  end
end
ActiveRecord::Associations::JoinHelper.send(:include, AccountJoinCondition)