On export, format dates to not include times

Currently all :type/DateTime values, including :type/Date values are
serialized as ISO timestamps, which includes the time and
timezone. This commit handles :type/Date fields differently, emitting
only the date (year, month and day).

Fixes #5828

src/metabase/api/dataset.clj

 (ns metabase.api.dataset
   "/api/dataset endpoints."
   (:require [cheshire.core :as json]
+            [clj-time.format :as tformat]
             [clojure.string :as str]
             [clojure.tools.logging :as log]
             [compojure.core :refer [POST]]
@@ -77,15 +78,41 @@
   (or (get-in ex/export-formats [export-format :context])
       (throw (Exception. (str "Invalid export format: " export-format)))))
 
+(defn- datetime-str->date
+  "Dates are iso formatted, i.e. 2014-09-18T00:00:00.000-07:00. We can just drop the T and everything after it since
+  we don't want to change the timezone or alter the date part."
+  [^String date-str]
+  (subs date-str 0 (.indexOf date-str "T")))
+
+(defn- swap-date-columns [date-col-indexes]
+  (fn [row]
+    (reduce (fn [acc idx]
+              (update acc idx datetime-str->date)) row date-col-indexes)))
+
+(defn- date-column-indexes
+  "Given `column-metadata` find the `:type/Date` columns"
+  [column-metadata]
+  (transduce (comp (map-indexed (fn [idx col-map] [idx (:base_type col-map)]))
+                   (filter (fn [[idx base-type]] (isa? base-type :type/Date)))
+                   (map first))
+             conj [] column-metadata))
+
+(defn- maybe-modify-date-values [column-metadata rows]
+  (let [date-indexes (date-column-indexes column-metadata)]
+    (if (seq date-indexes)
+      ;; Not sure why, but rows aren't vectors, they're lists which makes updating difficult
+      (map (comp (swap-date-columns date-indexes) vec) rows)
+      rows)))
+
 (defn as-format
   "Return a response containing the RESULTS of a query in the specified format."
   {:style/indent 1, :arglists '([export-format results])}
-  [export-format {{:keys [columns rows]} :data, :keys [status], :as response}]
+  [export-format {{:keys [columns rows cols]} :data, :keys [status], :as response}]
   (api/let-404 [export-conf (ex/export-formats export-format)]
     (if (= status :completed)
       ;; successful query, send file
       {:status  200
-       :body    ((:export-fn export-conf) columns rows)
+       :body    ((:export-fn export-conf) columns (maybe-modify-date-values cols rows))
        :headers {"Content-Type"        (str (:content-type export-conf) "; charset=utf-8")
                  "Content-Disposition" (str "attachment; filename=\"query_result_" (u/date->iso-8601) "." (:ext export-conf) "\"")}}
       ;; failed query, send error message

test/metabase/api/dataset_test.clj

 (ns metabase.api.dataset-test
   "Unit tests for /api/dataset endpoints."
-  (:require [cheshire.generate :as generate]
+  (:require [cheshire
+             [core :as json]
+             [generate :as generate]]
+            [clojure.data.csv :as csv]
             [dk.ative.docjure.spreadsheet :as spreadsheet]
             [expectations :refer :all]
             [medley.core :as m]
@@ -153,3 +156,34 @@
                                                ["ABC"]])
        (spreadsheet/select-sheet "Results")
        (spreadsheet/select-columns {:A "Values"})))
+
+(defn- parse-and-sort-csv [response]
+  (sort-by
+   ;; ID in CSV is a string, parse it and sort it to get the first 5
+   (comp #(Integer/parseInt %) first)
+   ;; First row is the header
+   (rest (csv/read-csv response))))
+
+;; Date columns should be emitted without time
+(expect
+  [["1" "2014-04-07" "5" "12"]
+   ["2" "2014-09-18" "1" "31"]
+   ["3" "2014-09-15" "8" "56"]
+   ["4" "2014-03-11" "5" "4"]
+   ["5" "2013-05-05" "3" "49"]]
+  (let [result ((user->client :rasta) :post 200 "dataset/csv" :query
+                (json/generate-string (wrap-inner-query
+                                        (query checkins))))]
+    (take 5 (parse-and-sort-csv result))))
+
+;; DateTime fields are untouched when exported
+(expect
+  [["1" "Plato Yeshua" "2014-04-01T08:30:00.000Z"]
+   ["2" "Felipinho Asklepios" "2014-12-05T15:15:00.000Z"]
+   ["3" "Kaneonuskatew Eiran" "2014-11-06T16:15:00.000Z"]
+   ["4" "Simcha Yan" "2014-01-01T08:30:00.000Z"]
+   ["5" "Quentin Sören" "2014-10-03T17:30:00.000Z"]]
+  (let [result ((user->client :rasta) :post 200 "dataset/csv" :query
+                (json/generate-string (wrap-inner-query
+                                        (query users))))]
+    (take 5 (parse-and-sort-csv result))))