t.belongs_to in migration

reference http://stackoverflow.com/questions/9471091/t-belongs-to-in-migration

auto create  *_id , *_type
       # Adds a reference. Optionally adds a +type+ column, if <tt>:polymorphic</tt> option is provided.

      # <tt>references</tt> and <tt>belongs_to</tt> are acceptable.

      # ===== Examples

      #  t.references(:goat)

      #  t.references(:goat, :polymorphic => true)

      #  t.belongs_to(:goat)

      def references(*args)

        options = args.extract_options!

        polymorphic = options.delete(:polymorphic)

        args.each do |col|

          @base.add_column(@table_name, "#{col}_id", :integer, options)

          @base.add_column(@table_name, "#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) unless polymorphic.nil?

        end

      end

      alias :belongs_to :references

Polymorphic Associations