history.clj

(defn timestamp
  "Returns object timestamped with the current system time in milliseconds"
  [obj millis]
  (assoc obj :timestamp (System/currentTimeMillis)))

(defn new-model-object
  "Returns new, uniquely identified, timestamped object with initial
  key-val pairs"
  [& key-vals]
  (timestamp (merge (apply hash-map kvs)
		    {:uuid (uuid)})))

(defn revisioned-update
  "Returns object with old state appended to value at :revisions key,
  and new, timestamped state, after applying (f obj & args) to the
  current state."
  [obj f & args]
  (let [obj1 (update-in obj [:revisions] conj obj)
	updated (apply f obj1 args)]
    (timestamp updated)))

(defn history
  "Returns all states of model object, oldest first"
  [obj]
  (sort-by :timestamp
	   (conj (:revisions obj)
		 (dissoc obj :revisions))))