Code for http://wp.me/pTHiC-10R, part 1

cars-original.clj

(require '[cheshire.core :refer [parse-string]])

(defn db-array [col] (reify java.sql.Array (getArray [_] (object-array col))))

(defn- json->data [data fields]
  (map (fn [data]
         (reduce (fn [data field]
                   (assoc data field (parse-string (get data field) true)))
                 data fields)) data))

(defn- discount-size [discount] 
  (if (or 
       (> (:amount discount) 10000)
       (> (:percent discount) 5)) 
    :high
    :normal))

(defn- jdbc-array-to-set
  "Convert a PostgreSQL JDBC4Array inside the map `m` - at the key `k` - into a se"
  [key m] (update-in m [key] #(some->> % (.getArray) (into #{}))))

(defn- compute-discount
  "Derive the :discount map based on the car's own discount and its active campaign, if applicable"
  [car]
  (let [{:keys [json campaign_discount_amount campaign_discount_percent]} car
        {:keys [discount_amount discount_percent]} json
        discount? (:use_campaign car)
        amount (if discount?
                 (apply + (remove nil? [discount_amount campaign_discount_amount]))
                 discount_amount)
        percent (if discount?
                  (apply + (remove nil? [discount_percent campaign_discount_percent]))
                  discount_percent)
        discount {:amount amount
                  :percent percent}
        discount-size (discount-size discount)
        ]
    (assoc car :discount discount :discount-size discount-size)))

(defn merge-campaigns
  "Given vector of car+campaign for a particular car, return a single car map
   with a selected campaign.
  "
  [car+campaigns]
  {:pre [(sequential? car+campaigns) :id]}
  (let [campaign-ks [:use_campaign :campaign_discount_amount :campaign_discount_percent :active]
        car (apply dissoc
                    (first car+campaigns)
                    campaign-ks)
        campaign (->> car+campaigns
                      (map #(select-keys % campaign-ks))
                      (filter :active)
                      (sort-by :use_campaign) ;; true, if any, will be last
                      last)]
    (assoc car :best-campaign campaign)))

(defn refine-cars
  "Process get-cars query result set - derive additional data, transform values into better ones
   There is one row per car and campaign, a car may have more campaigns - we pick the best one.
  "
  [cars-raw]
  (->> cars-raw
   (#(json->data % [:json]))
   (#(map merge-campaigns
        (vals (group-by :id %))))
   (map (comp            ;; reminder - the 1st fn is executed last
         compute-discount
         (fn [m] (update-in m [:keywords] (partial remove nil?))) ;; {NULL} => []
         (partial jdbc-array-to-set :keywords)
         (partial jdbc-array-to-set :category_ref)
         ))
   ))

(refine-cars [
              {:id 1
               :json "{\"discount_amount\":9000,\"discount_percent\":0}"
               :campaign_discount_amount 2000 
               :campaign_discount_percent nil
               :use_campaign false
               :active true
               :keywords (db-array ["fast"])
               :category_ref (db-array [])}])