datomic compose identifier

compose.edn

{:db/id #db/id[:db.part/db]
  :db/ident :createWithComposedID
  :db/fn #db/fn {:lang "clojure"
                 :params [db m compose-assoc existing-assoc parent-ident]
                 :code (let [parent (d/entity db parent-ident)
                             newRecord (merge m {:db/id (d/tempid :db.part/user)
                                                 compose-assoc (str (:db/id parent) ":" (existing-assoc m))})]
                                                 [new-record])}}

example.clj

(require '[datomic.api :as d])

(def conn (d/connection "datomic:mem://testdb"))

(def parent-id (ffirst (d/q '[:find ?e
                              :where [?e :parent/gid #uuid"blah"] (d/db conn))))

(d/transact conn [[:createWithComposedID {:product/number "1001A" 
                                          :product/price 13.50 
                                          :parent/_products parent-id} :product/id :product/number parent-id]])