undo-tx.clj

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

(defn undo-transact
  [db tx]
  (let [ref? (set (d/q '[:find [?ident ...] :where [?e :db/ident ?ident] [?e :db/valueType :db.type/ref]] db))
        ident (into {} (d/q '[:find ?id ?ident :where [?id :db/ident ?ident]] db))
        hist (d/history db)
        data-from-tx (d/q '[:find ?e ?a ?v ?tx ?op
                            :in $ ?tx
                            :where
                            [?i :db/ident ?a]
                            [?e ?i ?v ?tx ?op]]
                          hist tx)
        tx-data (for [[e a v tx op] data-from-tx]
                  [e a (if (ref? a) (ident v v) v) tx op])
        tx-data (keep (fn [[e a v tx op]]
                        (when-not (= e tx)
                          [(if op :db/retract :db/add) e a v])) tx-data)]
    (into [[:db/add (d/tempid :db.part/tx) :audit/undo-tx tx]] tx-data)))