full-outer-join.clj

(defn full-outer-join
  "merge two collections on key. Equivalent to SQL full outer join.
  assumes keys are unique"
  [k A B]
  (let [A' (->> A
                (group-by k)
                (map (fn [[k v]] [k (first v)]))
                (into {}))
        B' (->> B
                (group-by k)
                (map (fn [[k v]] [k (first v)]))
                (into {}))]
    (map (fn [[k v]] v)
         (merge-with merge A' B'))))

(comment
  (def A [{:k 1 :foo "bar"}
          {:k 2 :foo "bla"}])

  (def B [{:k 2 :wtv 123}
          {:k 3 :wtv 456}])
  (full-outer-join :k A B)
  ({:k 1, :foo "bar"}
   {:k 2, :foo "bla", :wtv 123}
   {:k 3, :wtv 456}))