has_one (ActiveRecord::Associations::ClassMethods)

    # File lib/active_record/associations.rb, line 925
          def has_one(association_id, options = {})
            if options[:through]
              reflection = create_has_one_through_reflection(association_id, options)
              association_accessor_methods(reflection, ActiveRecord::Associations::HasOneThroughAssociation)
            else
              reflection = create_has_one_reflection(association_id, options)
              association_accessor_methods(reflection, HasOneAssociation)
              association_constructor_method(:build,  reflection, HasOneAssociation)
              association_constructor_method(:create, reflection, HasOneAssociation)
              configure_dependency_for_has_one(reflection)
            end
          end