references

# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 312
      def references(*args)
        options = args.extract_options!
        polymorphic = options.delete(:polymorphic)
        index_options = options.delete(:index)
        foreign_key_options = options.delete(:foreign_key)
        type = options.delete(:type) || :integer

        if polymorphic && foreign_key_options
          raise ArgumentError, "Cannot add a foreign key on a polymorphic relation"
        end

        args.each do |col|
          column("#{col}_id", type, options)
          column("#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) if polymorphic
          index(polymorphic ? %(type id).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options.is_a?(Hash) ? index_options : {}) if index_options
          if foreign_key_options
            to_table = Base.pluralize_table_names ? col.to_s.pluralize : col.to_s
            foreign_key(to_table, foreign_key_options.is_a?(Hash) ? foreign_key_options : {})
          end
        end
      end