foo.clj

(require '[datomic.client.api :as d])
(let [conn (-> {:server-type :dev-local
                :system      "hello"}
             d/client
             (doto (d/delete-database {:db-name "hello"})
                   (d/create-database {:db-name "hello"}))
             (d/connect {:db-name "hello"}))
      do-tx! (fn [s] (-> conn
                       (d/transact {:tx-data [{:db/ident :hello
                                               :db/doc   s}
                                              {:db/id  "datomic.tx"
                                               :db/doc s}]})
                       :tempids
                       (get "datomic.tx")))
      q! (fn [t]
           (first  (d/q '[:find ?doc :where [?e :db/ident :hello] [?e :db/doc ?doc]]
                     (d/as-of (d/db conn) t))))
      first-tx (do-tx! "first one")
      second-tx (do-tx! "second one")
      last-tx (do-tx! "last one")]
  (q! last-tx)
  (q! (dec  last-tx)))