scratch.clj

;; reduce is the key here
(def s [{:pk 1 :data "a"} {:pk 2 :data "b"} {:pk 3 :data "c"}])
(def h (reduce (fn [result item] (merge result {(:pk item) item})) {} s))

;; anonymous function version
(def h2 (reduce #(merge %1 {(:pk %2) %2}) {} s)) 
;; h is {3 {:pk 3, :data "c"}, 2 {:pk 2, :data "b"}, 1 {:pk 1, :data "a"}}

;; general form
(defn m-of-ms
  "given a key and seq of maps create a map of maps index by key"
  [k s]
  (reduce #(merge %1 {(k %2) %2}) {} s))