Project a map onto a tree of keys

projection.clj

(defn project-paths [projection]
  (loop [pr projection
         paths '[]]
    (let [[el next] pr]
      (if (empty? pr)
        paths

        (if (vector? next)
          (recur
           (drop 2 pr)
           (concat paths (map #(apply vector el %) (find-paths next))))

          (recur
           (rest pr)
           (conj paths [el])))))))

(find-paths [:a [:b1 [:c1 [:d1 :a2]]] :b [:c :d]
               'a 'b 'c])

(def m {:a {:b "foo" :c {:d "1234"}} :x "456" :y 1 :z 2})

(reduce (fn [m' p]
          (assoc-in m' p (get-in m p)))
        {}
        (project-paths [:a [:b] :y]))