Check 'POST /items' payload is application/json

This commit adds a middleware function that checks any POST endpoint is
called with Content-Type: application/json. Returns `HTTP 415
Unsupported Media Type` if this is not the case.

TODO: this commit does not check that the payload is actually
well-formed JSON. If the payload is *not* valid JSON, muutaja will fail,
which will result in an `HTTP 500 Server error`.

src/yohoho/core.clj

@@ -6,6 +6,8 @@
   (:gen-class))
 
 
+
+;; Handlers -------------------------------------------------------
 (defn ahoy-handler
   [_]
   {:status 200
@@ -29,6 +31,22 @@
      :body {:id id, :name name, :email email}}))
 
 
+;; Custom middleware ----------------------------------------------
+(defn wrap-content-type-json
+  "Middleware that checks is a POST request is bring sent data as JSON"
+  [handler]
+  (fn [request]
+    (let [http-verb (:request-method request)
+          is-post? (= :post http-verb)
+          content-type (get-in request [:headers "content-type"])
+          is-json? (= "application/json" content-type)]
+      (cond (not is-post?) (handler request)
+            (and is-post? is-json?) (handler request)
+            :else {:status 415
+                   :body {:error "Unsupported Media Type"
+                          :message "Content-Type must be application/json"}}))))
+
+;; Routing --------------------------------------------------------
 (def app
   (ring/ring-handler
    (ring/router
@@ -37,13 +55,15 @@
                 :post post-items-handler}]]
     ;; Use muuntaja middleware to automatically decode JSON
     {:data {:muuntaja m/instance
-            :middleware [muuntaja/format-middleware]}})
+            :middleware [wrap-content-type-json
+                         muuntaja/format-middleware]}})
    ;; Default route: anything not explicitely handled should give a 404
    (ring/create-default-handler
     {:not-found (constantly {:status 404
                              :body {:error "Not Found"}})})))
 
 
+;; Entry point ----------------------------------------------------
 (defn -main
   "HolidayPirates take-home assignement.
    Goal: build a (very) small REST API that exposes to endpoints."