core.clj

(ns core)

(defn- keyed-by [coll f]
  (into {} (map (fn [x] [(f x) x]) coll)))

(defn- vconj [coll & xs]
  (vec (apply conj coll xs)))

(def people [{:name "Jeff"
              :instrument "Mbira"}
             {:name "Raven"
              :instrument "Drums"}
             {:name "Coby"
              :instrument "Bass"}])

(def songs [{:name "Brand New Day"
             :writer "Jeff"}
            {:name "Piece of Mind"
             :writer "Jeff"}
            {:name "Nearly No"
             :writer "Coby"}])

(defn merge-related [originators belongings from to unite]
  (let [keyed (keyed-by originators from)]
    (vals (reduce (fn [origs belonging]
                    (update origs (to belonging) unite belonging))
                  keyed
                  belongings))))

(defn people->songwriters [ppl songs]
  (merge-related ppl songs :name :writer (fn [writer song]
                                           (update writer :songs vconj (:name song)))))

core_test.clj

(ns lando-clojure.handler-test
  (:require [clojure.test :refer :all]
            [core]))

(deftest merges-maps-together
  (is (= [{:name "Jeff"
           :instrument "Mbira"
           :songs ["Brand New Day" "Piece of Mind"]}
          {:name "Raven"
           :instrument "Drums"}
          {:name "Coby"
           :instrument "Bass"
           :songs ["Nearly No"]}]
         (core/people->songwriters subject/people subject/songs))))