model.clj

(def input
  {:tracker [{:cid 1 :foo "b"} {:cid 2 :bar "d"}]
   :images  [{:cid 1 :baz "a"} {:cid 1 :bar "abc"} {:cid 2 :boo "e"}]})

(def desired-output
  {1 {:tracker [{:foo "b"}] :images [{:baz "a"} {:bar "abc"}]}
   2 {:tracker [{:bar "d"}] :images [{:boo "e"}]}})

; Solution h/t Paulus Esterhazy
(defn mrg [kk input]
  (reduce
    (fn [m [k v]] (update-in m k #(conj % v)))
    {}
    (for [[table rows] input row rows]
      [[(kk row) table] (dissoc row kk)])))

;; (map (fn [c]
;;        (reduce (fn [m]
;;               (assoc-in c [(key m)] ((:campaign_id c) (val m))))
;;             data-to-merge))
;;        campaigns)))

(group-by (comp :cid second) 
          (for [[k vs] input 
                v vs] 
               [k v]))

;; is a very decent start

(defn mrg [input]
  (reduce
    (fn [m [k v]] (update-in m k #(merge % v)))
    {}
    (for [[table rows] input row rows] [[(:cid row) table] (dissoc row :cid)])))