Using Datomic's d/with to query a database as though some transaction had been committed, before actually committing it!

with.clj

(def conn (d/conn some-uri))

(def some-id 12345)

(defn get-v [db] (ffirst (d/q '[:find ?v :in $ ?e :where [?e :some/attribute ?v]] db some-id)))

(get-v (d/db conn))
;; => :some-value

(def tx [[:db/retract some-id :some/attribute :some-value]
         [:db/add some-id :some/attribute :some-other-value]])

;; try the tx with d/with
(def updated-db (-> (d/db conn) (d/with tx) :db-after))

(get-v updated-db)
;; => :some-other-value

;; db hasn't changed!
(get-v (d/db conn))
;; => :some-value

;; commit
(d/transact conn tx)

;; now it has!
(get-v (d/db conn))
;; => :some-other-value