Group a map by one or more keys

group-by-keys.clj

(defn group-by-keys
  "Group maps by a set of keys. For each key not grouped, all values are
  returned as a set"
  [ms kys]
  (let [kys (set kys)
        grouped (group-by #(select-keys % kys) ms)]
    (map (fn [[pk pv]]
           (into pk
                 (reduce (fn [a [k v]] (update a k #(conj (or % #{}) v)))
                         {}
                         (->> pv
                              (mapcat seq)
                              (remove (comp kys first))))))
         grouped)))