Upsert on entity with identity composite tuple

[fahtative]

This is also mentioned on Datascript's Issues.
Basically upsert to update fails on entity with unique composite tuple:

(def schema {:a+b {:db/tupleAttrs [:a :b]
                     :db/unique :db.unique/identity}})
(def conn (dl/get-conn "test-dl" schema))
(dl/transact conn [{:a 1 :b 2}])
(dl/transact conn [{:a 1 :b 2}])

Execution error (ExceptionInfo) at datalevin.db/validate-datom (db.clj:724).
Cannot add #datalevin/Datom [2 :a+b [1 2]] because of unique constraint: [#datalevin/Retrieved [nil 3 [1 2] nil] #datalevin/Retrieved [1 nil nil 0]]

[huahaiy]

Will take a look.