datomic isComponent

user.clj

 (do
  (d/delete-database "datomic:mem://test-is-component")
  (d/create-database "datomic:mem://test-is-component")
  (let [conn (d/connect "datomic:mem://test-is-component")
        db (-> (d/transact conn [{:db/ident :dev/ref,
                                   :db/valueType :db.type/ref
                                   :db/cardinality :db.cardinality/many
                                   :db/isComponent true}])
               deref
               :db-after)
        {:keys [db-after tempids]} (d/with db [{:db/id "parent1"
                                                :db/doc "parent1"
                                                :dev/ref "child"}
                                               {:db/id "parent2"
                                                :db/doc "parent2"}
                                               {:db/id "child"
                                                :db/doc "child"}
                                               ])
        db-a db-after
        ; move child to parent2?
        db-b (:db-after (d/with db-a [[:db/add
                                       (get tempids "parent2")
                                       :dev/ref
                                       (get tempids "child")]]))]
    ; child's parent is still parent1
    (assert (= (:dev/_ref (d/entity db-b (get tempids "child")))
               (d/entity db-b (get tempids "parent1"))))
    ; parent1 and parent2 both "own" child
    (assert (= (:dev/ref (d/entity db-b (get tempids "parent1")))
               (:dev/ref (d/entity db-b (get tempids "parent2")))))
    ))