gistfile1.clj

(defn not-set
  "Returns true if attr is not set for e"
  [db e attr]
  (let [result (api/q '[:find ?v
                        :in $ ?e ?a
                        :where [?e ?a ?v]]
                      db e attr)]
    (-> result first empty?)))

(defn pets-without-tails [owner-id]
  (api/q '[:find ?e
           :in $ ?owner-id
           :where
           [?e :pet/owner ?owner-id]
           [(my-ns/not-set $ ?e :pet/tail)]]
         (db/db) owner-id))