join.clj

   (def data {:cards
              [{:id "card-1", :idList "list-id-1", :name "Task 1"}],
              :lists
              [{:id "list-id-1", :name "List 1"}
               {:id "list-id-2", :name "List 2"}]})

   (vec (o/for-query
          (o/and
            (d/query-in data _ [:lists _] ?list-item)
            (d/query data ?list-item :id ?list-id)
            (d/query data ?list-item :name ?list-name)
            (o/project
              ;; tad ugly, need to fix this let
              (let [list-id ?list-id]
                (vec (o/for-query
                       (o/and
                         (d/query-in data ?card [:idList] list-id)
                         (d/query data ?card :name ?card-name)
                         (d/query data ?card :id ?card-id))
                       {:id ?card-id
                        :name ?card-name}))) ?cards))

          {:id   ?list-id
           :name ?list-name
           :cards ?cards}))

   ;; [{:id "list-id-1", :name "List 1", :cards [{:id "card-1", :name "Task 1"}]}
   ;;  {:id "list-id-2", :name "List 2", :cards []}]