create_belongs_to_reflection

# File activerecord/lib/active_record/associations.rb, line 1773
        def create_belongs_to_reflection(association_id, options)
          options.assert_valid_keys(valid_keys_for_belongs_to_association)
          reflection = create_reflection(:belongs_to, association_id, options, self)

          if options[:polymorphic]
            reflection.options[:foreign_type] ||= reflection.class_name.underscore + "_type"
          end

          reflection
        end