pathom.clj

(let [conn (doto (ds/create-conn {:user/addresses {:db/valueType   :db.type/ref
                                                   :db/cardinality :db.cardinality/many} 
                                  :user/id        {:db/unique :db.unique/identity}})
             (ds/transact! [{:user/id        1
                             :user/addresses [-1 -2]}
                            {:user/id        2
                             :user/addresses [-2 -3]}
                            {:user/id        3
                             :user/addresses [-4]}
                            {:db/id          -1
                             :address/id     1
                             :address/street "one"}
                            {:db/id          -2
                             :address/id     2
                             :address/street "two"}
                            {:db/id          -3
                             :address/id     3
                             :address/street "3"}
                            {:db/id          -4
                             :address/id     4
                             :address/street "four"}]))
      register [(pc/resolver `street
                             {::pc/input  #{:address/id}
                              ::pc/batch? true
                              ::pc/output [:address/street]}
                             (fn [{::keys [db]} addresses]
                               (prn [:street addresses])
                               (let [ids (map :address/id addresses)
                                     id->user (into {}
                                                    (ds/q '[:find ?id (pull ?e [:address/street])
                                                            :in $ [?id ...]
                                                            :where
                                                            [?e :address/id ?id]]
                                                          db))]
                                 (map id->user ids))))
                (pc/resolver `addresses
                             {::pc/input  #{:user/id}
                              ::pc/batch? true
                              ::pc/output [:user/addresses]}
                             (fn [{::keys [db]} users]
                               (prn [:addresses users])
                               (let [ids (map :user/id users)
                                     id->user (into {}
                                                    (ds/q '[:find ?id (pull ?e [:user/id
                                                                                {:user/addresses [:address/id]}])
                                                            :in $ [?id ...]
                                                            :where
                                                            [?e :user/id ?id]]
                                                          db))]
                                 (map id->user ids))))
                (pc/resolver `all-users
                             {::pc/output [:app/users]}
                             (fn [{::keys [db]} _]
                               (prn [:all-users _])
                               {:app/users (ds/q '[:find [(pull ?e [:user/id]) ...]
                                                   :where
                                                   [?e :user/id ?id]]
                                                 db)}))]
      parser (p/parser {::p/plugins [(pc/connect-plugin {::pc/register register})]})
      env {::p/reader               [p/map-reader
                                     pc/reader3
                                     pc/open-ident-reader
                                     p/env-placeholder-reader]
           ::db                     (ds/db conn)
           ::p/placeholder-prefixes #{">"}}]

  (parser env [{:app/users
                [:user/id
                 {:user/addresses [:address/id
                                   :address/street]}]}]))
(comment
  [:all-users {}]
[:addresses ({:user/id 2} {:user/id 3} {:user/id 1})]
[:street ({:address/id 2} {:address/id 3})]
[:street ({:address/id 4})]
[:street ({:address/id 1} {:address/id 2})]
=>
{:app/users [{:user/id 2, :user/addresses [{:address/id 2, :address/street "two"} {:address/id 3, :address/street "3"}]}
             {:user/id 3, :user/addresses [{:address/id 4, :address/street "four"}]}
             {:user/id 1,
              :user/addresses [{:address/id 1, :address/street "one"} {:address/id 2, :address/street "two"}]}]})