artdent

(defn ways-by-node [ways]
  (reduce
   (partial merge-with (comp vec concat))
   (for [way ways, id (:nodes way)]
     (hash-map id [(:id way)]))))

artdent

(defn collect-ways-by-node [ways]
  (loop [ways ways
         accum {}]
    (if (seq ways)
      (let [{:keys [nodes id]} (first ways)
            new-accum (zipmap nodes (map vector (repeat id)))]
        (recur (rest ways) (merge-with (comp vec concat) accum new-accum)))
      accum)))