(defclass user ()
  ((id :type integer :initarg :id)
  (name :type string :initarg :name)))

(defmethod jonathan:%to-json ((user user))
  (jonathan:with-object
  (jonathan:write-key-value "id" (slot-value user 'id))
  (jonathan:write-key-value "name" (slot-value user 'name))))

(jonathan:to-json (make-instance 'user :id 1 :name "Rudolph"))
=> "{\"id\":1,\"name\":\"Rudolph\"}"