core.clj

(def hh [{:a 1, :b 2, :c 3 :aa 4} {:a 1, :b 2, :c 4 :aa 4} {:a 99, :b 2, :c 3 :aa 5}])
(def rname :trial-ups)
(def main-f [:b :a])

;; first stab, we pivot over the keys of the "root" entity,
;; group by them, and strip the rest out
(defn hydrate-with-main-entity-keys [m ks as]
  (->> (group-by  #(select-keys % main-f) hh)
       (m/map-vals (partial map #(apply dissoc % main-f)))
       (map (fn [[k v]] (assoc k rname (vec v))))))

;; wait,... it would be good to be able to "strip" the child tables instead knowing which is the common part
(defn other-keys [m ks]
  (keys (apply dissoc m ks)))

(defn hydrate-keys [m ks as]
  (->> (group-by  #(select-keys % (other-keys % ks)) m)
       (m/map-vals (partial map #(apply dissoc % (other-keys % ks))))
       (map (fn [[k v]] (assoc k as (vec v))))))

;; select-keys and dissoc are kind of opposite. instead of other-keys, we can just swap them
(defn hydrate-keys [m ks as]
  (->> (group-by  #(apply dissoc % ks) m)
       (m/map-vals (partial map #(select-keys % ks)))
       (map (fn [[k v]] (assoc k as (vec v))))))

(hydrate-keys hh [:aa] :my-as)

(hydrate-keys
 (hydrate-keys hh [:aa] :my-as)
 [:c]
 :my-cs)

(reduce #(apply hydrate-keys %1 %2)
        hh
        [[[:aa] :my-as] [[:c] :my-cs]])