Datomic db functions

number-generation-example.clj

{:db/id #db/id [:db.part/user]
 :db/ident :generate-number
 :db/fn #db/fn {:lang "clojure"
                :params [db
                         counter-e counter-a
                         prefix
                         target-e target-a]
                :code (let [current (or (ffirst (q '[:find ?v
                                                     :in $ ?e ?a
                                                     :where [?e ?a ?v]]
                                                     db counter-e counter-a))
                                        1)]
                        [[:db/add counter-e counter-a (inc current)]
                         [:db/add target-e target-a
                          (str prefix
                              (format "%07d" current))]])}}