datomic backreferences where iscomponent=true

gistfile1.clj

(def uri "datomic:mem://my-db")
(d/create-database uri)

(def conn  (d/connect uri))

(d/transact conn [
                   {:db/id #db/id [:db.part/db]
                    :db/ident :entity/name
                    :db/valueType :db.type/string
                    :db/isComponent true
                    :db/cardinality :db.cardinality/one
                    :db/doc ""
                    :db.install/_attribute :db.part/db}

                   {:db/id #db/id [:db.part/db]
                    :db/ident :entity/one
                    :db/valueType :db.type/ref
                    :db/isComponent true
                    :db/cardinality :db.cardinality/one
                    :db/doc ""
                    :db.install/_attribute :db.part/db}
                   ])


(d/transact conn [{:db/id (d/tempid :db.part/user)
                   :entity/name "parent 1"
                   :entity/one {:db/id (d/tempid :db.part/user -1)
                                :entity/name "child"}}
                  {:db/id (d/tempid :db.part/user)
                   :entity/name "parent 2"
                   :entity/one {:db/id (d/tempid :db.part/user -1)
                                :entity/name "child"}}])

(def db (d/db conn))


;; confirm 2 parents ref the same child
(d/q '[:find ?parent ?one-child :where [?parent :entity/one ?one-child]] db)


;;; nav via back references. the result is not a set but a single entity (one of the 'parent' entities) because the attribute is 
;;; ':db/isComponent true'

(def one-child (ffirst (d/q '[:find ?one-child :where [_ :entity/one ?one-child]] db)))

(->> (d/entity db one-child)
     :entity/_one)