cars-simplified-original.clj

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

(defn- json->data [key m]
  (update-in m [key] #(parse-string % true)))
  
(defn- select-campaign [car+campaigns]
  (first car+campaigns))

(defn- jdbc-array-to-set
  [key m]
  (update-in m [key] #(apply hash-set (.getArray %))))

(defn refine-cars-simple
  "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
   (map (partial json->data :json)) ;; <- this I originally forgot
   ;; group rows for the same car => [[car1 ..][car2 ..]]
   (group-by :id)
   (vals)
   ;; join all car1 vectors into one car ..
   (map select-campaign)
   (map (fn [car]
          (->>
           car
           (jdbc-array-to-set :category_ref)
           (jdbc-array-to-set :keywords))))))

cars-simplified-refactored.clj

(require '[cheshire.core :refer [parse-string]])
(require '[clojure.set :refer [subset? difference]])

(defn- car? [{:keys [id] :as car}]
  (and (map? car) id))

(defn- json->data [key m]
  {:pre [(contains? m key) (string? (get m key))], :post [(map? (get % key))]}
  (update-in m [key] parse-string true))
  
(defn- select-campaign [[first-car :as all]]
  {:pre [(sequential? all) (car? first-car)], :post [(car? %)]}
  first-car)

(defn- jdbc-array-to-set
  [key m]
  {:pre [(contains? m key) (instance? java.sql.Array (get m key))], :post [(set? (get % key))]}
  (update-in m [key] #(apply hash-set (.getArray %))))

(defn group-rows-by-car [cars-raw]
  {:pre [(sequential? cars-raw) (car? (first cars-raw))]
   :post [(sequential? %) (vector? (first %))]}
  (vals (group-by :id cars-raw)))

(defn refine-car [car]
  {:pre [(car? car) (:keywords car) (:category_ref car)]}
  (->> car
    (jdbc-array-to-set :category_ref)
    (jdbc-array-to-set :keywords)))

(defn refine-cars-simple
  "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
   (map (partial json->data :json)) ;; <- this I originally forgot
   (group-rows-by-car)
   (map select-campaign)
   (map refine-car)))

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

(refine-cars-simple [{:id 1, :json "{\"discount\":5000}", :campaign_discount 3000, :category_ref (empty-array), :keywords (empty-array)}])

A simplified example for http://theholyjava.wordpress.com/2014/04/30/clojure-how-to-prevent-expected-map-got-vector-and-similar-errors/