Clojure, merging two array of maps

Question

I have two arrays of maps

1st is [{:a 1 :b 2 :d 6} {:a 2 :b 2} {:a 7 :b 7}]

2nd is [{:a 3 :c 3 :e 9 :y 7} {:a 2 :b 6 :c 8}]

depending on the value of a i.e. if its matches in 2nd array the '2nd map' should be merged with '1st map' and the resultant array of maps should be

Res should be [{:a 1 :b 2 :d 6} {:a 2 :b 6 :c 8} {:a 7 :b 7} {:a 3 :c 3 :e 9 :y 7}]

Can anyone help me on this. Thanks in advance.

Answer 1

Here you go:

user> (def xs [{:a 1 :b 2 :d 6} {:a 2 :b 2} {:a 7 :b 7}])
#'user/xs
user> (def ys  [{:a 3 :c 3 :e 9 :y 7} {:a 2 :b 6 :c 8}])
#'user/ys
user> (for [[a ms] (group-by :a (concat xs ys))] (apply merge ms))
({:a 1, :b 2, :d 6} {:a 2, :c 8, :b 6} {:a 7, :b 7} {:y 7, :a 3, :c 3, :e 9})

Answer 2

This data structure looks very unwieldy to me nevertheless here's my take:

(defn key-by-a [coll]
  "Convert a list of maps to a map of maps keyed by their vals at :a"
  (apply hash-map (mapcat (juxt :a identity) coll)))

(defn merge-map-lists [l1 l2]
  (->> [l1 l2]
    (map key-by-a)
    (apply merge-with merge)
    (vals)))

One thing it doesn't do is maintaining order of the input lists but since it is not clear which list decides (both might have same keys in different orders) I left that out.