Atomic find-or-create in datomic

gistfile1.clj

;; Schema

{:db/doc "Find or create tag atomically."
   :db/ident :find-or-create-tag
   :db/fn #db/fn {:lang "clojure"
                  :params [mydb n key value]
                  :code (when (empty? (d/q '[:find ?e :in $ ?ns ?key ?value :where
                                             [?e :meta/tag-namespace ?n]
                                             [?e :meta/tag-key ?key]
                                             [?e :meta/tag-value ?value]]
                                           mydb n key value))
                          [{:db/id (d/tempid :cognician)
                            :meta/tag-namespace n
                            :meta/tag-key key
                            :meta/tag-value value}])}
   :db/id #db/id [:db.part/db]}

;; Application fn

(defn find-or-create-tag!
  [tag-ns tag-key tag-value]
  (if-let [tag (find-tag tag-ns tag-key tag-value)]
    tag
    (do
      (db/transact! [[:find-or-create-tag tag-ns tag-key tag-value]])
      (find-tag tag-ns tag-key tag-value))))