Datomic: Getting the id of an inserted entity

gistfile1.clj

;; see http://stackoverflow.com/questions/17190334/getting-the-id-of-an-inserted-entity-in-diatomic
(require '[datomic.api :as d])
(def uri "datomic:mem://test")
(d/create-database uri)
(def conn (d/connect uri))

;; create an atribute
(d/transact conn [{:db/id #db/id[:db.part/db]
                   :db/ident :some/attribute
                   :db/valueType :db.type/string
                   :db/cardinality :db.cardinality/one
                   :db.install/_attribute :db.part/db}])

;; transact multiple entities with that attribute
(def tx @(d/transact conn
                     [[:db/add #db/id [:db.part/user -100] :some/attribute "A"]
                      [:db/add #db/id [:db.part/user -200] :some/attribute "B"]
                      [:db/add #db/id [:db.part/user -300] :some/attribute "C"]]))

;; get back the real entity id from a temporary id
(def bEid (d/resolve-tempid (d/db conn) (:tempids tx) (d/tempid :db.part/user -200)))
;; 17592186045440. Go from -200 (tempid) to 17592186045440 (real entity id)

;; verify you got the right one
(:some/attribute (d/entity (d/db conn) bEid))
;; "B"

Would it be better to use (:db-after tx) instead of (d/db conn)?