resolver.clj

(defn username-by-id
  {::pc/kind   pc/resolver
   ::pc/input  #{:app.user/id}
   ::pc/output [:app.user/username]}
  [{::keys [db]} {:app.user/keys [id]}]
  {:app.user/username (-> (j/query db ["SELECT username FROM app_user WHERE id = ?"
                                       id])
                          first
                          :username)})

(let [var->resolve (fn [x]
                     (let [sym (symbol x)
                           {::pc/keys [kind] :as options} (meta x)]
                       (kind sym options @x)))]
  (var->resolve #'username-by-id))