keep-keys-on-join.clj

(let [keep-keys-on-join (fn [ks]
                          {::p/wrap-read (fn [reader]
                                           (fn [env]
                                             (let [result (reader env)
                                                   join? (-> env :ast :type (= :join))
                                                   base-map (when join?
                                                              (select-keys (p/entity env) ks))
                                                   merge-with-base (fn [e]
                                                                     (if (map? e)
                                                                       (merge-with p/entity-value-merge base-map e)
                                                                       e))
                                                   join-many? (and join?
                                                                   (not (empty? base-map))
                                                                   (vector? result))]
                                               (cond
                                                 join-many? (mapv merge-with-base result)
                                                 join? (merge-with-base result)
                                                 :default result))))})
      register [(pc/resolver `friends {::pc/input  #{:root}
                                       ::pc/output [:friends]}
                             (fn [_ _]
                               {:friends [{:id 2}]}))]
      parser (p/parser {::p/plugins [(keep-keys-on-join [:root])
                                     (pc/connect-plugin {::pc/register register})]})]
  (parser {::p/reader               [p/map-reader
                                     pc/reader2
                                     pc/ident-reader
                                     p/env-placeholder-reader]
           ::p/placeholder-prefixes #{">"}}
          [{[:root 5555] [:root
                          {:friends [:root]}]}]))