create_has_one_reflection

# File activerecord/lib/active_record/associations.rb, line 1227
        def create_has_one_reflection(association_id, options)
          options.assert_valid_keys(
            :class_name, :foreign_key, :remote, :conditions, :order, :include, :dependent, :counter_cache, :extend, :as
          )

          create_reflection(:has_one, association_id, options, self)
        end