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Ring Coercion

Basic coercion is explained in detail in the Coercion Guide. With Ring, both request parameters and response bodies can be coerced.

The following request parameters are currently supported:

typerequest source
:query:query-params
:body:body-params
:request:body-params, allows per-content-type coercion
:form:form-params
:header:header-params
:path:path-params
:multipart:multipart-params, see Default Middleware

To enable coercion, the following things need to be done:

  • Define a reitit.coercion/Coercion for the routes
  • Define types for the parameters and/or responses
  • Mount Coercion Middleware to apply to coercion
  • Use the coerced parameters in a handler/middleware

Define coercion​

reitit.coercion/Coercion is a protocol defining how types are defined, coerced and inventoried.

Reitit ships with the following coercion modules:

Coercion can be attached to route data under :coercion key. There can be multiple Coercion implementations within a single router, normal scoping rules apply.

Defining parameters and responses​

Parameters are defined in route data under :parameters key. It's value should be a map of parameter :type -> Coercion Schema.

Responses are defined in route data under :responses key. It's value should be a map of http status code to a map which can contain :body key with Coercion Schema as value.

Below is an example with Plumatic Schema. It defines schemas for :query, :body and :path parameters and for http 200 response :body.

Handlers can access the coerced parameters via the :parameters key in the request.

(require '[reitit.coercion.schema])
(require '[schema.core :as s])

(def PositiveInt (s/constrained s/Int pos? 'PositiveInt))

(def plus-endpoint
  {:coercion reitit.coercion.schema/coercion
   :parameters {:query {:x s/Int}
                :body {:y s/Int}
                :path {:z s/Int}}
   :responses {200 {:body {:total PositiveInt}}}
   :handler (fn [{:keys [parameters]}]
              (let [total (+ (-> parameters :query :x)
                             (-> parameters :body :y)
                             (-> parameters :path :z))]
                {:status 200
                 :body {:total total}}))})

Coercion Middleware​

Defining a coercion for a route data doesn't do anything, as it's just data. We have to attach some code to apply the actual coercion. We can use the middleware from reitit.ring.coercion:

  • coerce-request-middleware to apply the parameter coercion
  • coerce-response-middleware to apply the response coercion
  • coerce-exceptions-middleware to transform coercion exceptions into pretty responses

Full example​

Here is a full example for applying coercion with Reitit, Ring and Schema:

(require '[reitit.ring.coercion :as rrc])
(require '[reitit.coercion.schema])
(require '[reitit.ring :as ring])
(require '[schema.core :as s])

(def PositiveInt (s/constrained s/Int pos? 'PositiveInt))

(def app
  (ring/ring-handler
    (ring/router
      ["/api"
       ["/ping" {:name ::ping
                 :get (fn [_]
                        {:status 200
                         :body "pong"})}]
       ["/plus/:z" {:name ::plus
                    :post {:coercion reitit.coercion.schema/coercion
                           :parameters {:query {:x s/Int}
                                        :body {:y s/Int}
                                        :path {:z s/Int}}
                           :responses {200 {:body {:total PositiveInt}}}
                           :handler (fn [{:keys [parameters]}]
                                      (let [total (+ (-> parameters :query :x)
                                                     (-> parameters :body :y)
                                                     (-> parameters :path :z))]
                                        {:status 200
                                         :body {:total total}}))}}]]
      {:data {:middleware [rrc/coerce-exceptions-middleware
                           rrc/coerce-request-middleware
                           rrc/coerce-response-middleware]}})))

Valid request:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "1"}
      :body-params {:y 2}})
; {:status 200, :body {:total 6}}

Invalid request:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "abba"}
      :body-params {:y 2}})
; {:status 400,
;  :body {:schema {:x "Int", "Any" "Any"},
;         :errors {:x "(not (integer? \"abba\"))"},
;         :type :reitit.coercion/request-coercion,
;         :coercion :schema,
;         :value {:x "abba"},
;         :in [:request :query-params]}}

Invalid response:

(app {:request-method :post
      :uri "/api/plus/3"
      :query-params {"x" "1"}
      :body-params {:y -10}})
; {:status 500,
;  :body {:schema {:total "(constrained Int PositiveInt)"},
;         :errors {:total "(not (PositiveInt -6))"},
;         :type :reitit.coercion/response-coercion,
;         :coercion :schema,
;         :value {:total -6},
;         :in [:response :body]}}

Per-content-type coercion​

You can also specify request and response body schemas per content-type. These are also read by the OpenAPI feature when generating api docs. The syntax for this is:

(def app
  (ring/ring-handler
   (ring/router
    ["/api"
     ["/example" {:post {:coercion reitit.coercion.schema/coercion
                         :request {:content {"application/json" {:schema {:y s/Int}}
                                             "application/edn" {:schema {:z s/Int}}
                                             ;; default if no content-type matches:
                                             :default {:schema {:yy s/Int}}}}
                         :responses {200 {:content {"application/json" {:schema {:w s/Int}}
                                                    "application/edn" {:schema {:x s/Int}}
                                                    :default {:schema {:ww s/Int}}}}}
                         :handler ...}}]]
    {:data {:middleware [rrc/coerce-exceptions-middleware
                         rrc/coerce-request-middleware
                         rrc/coerce-response-middleware]}})))

Pretty printing spec errors​

Spec problems are exposed as is in request & response coercion errors. Pretty-printers like expound can be enabled like this:

(require '[reitit.ring :as ring])
(require '[reitit.ring.middleware.exception :as exception])
(require '[reitit.ring.coercion :as coercion])
(require '[expound.alpha :as expound])

(defn coercion-error-handler [status]
  (let [printer (expound/custom-printer {:theme :figwheel-theme, :print-specs? false})
        handler (exception/create-coercion-handler status)]
    (fn [exception request]
      (printer (-> exception ex-data :problems))
      (handler exception request))))

(def app
  (ring/ring-handler
    (ring/router
      ["/plus"
       {:get
        {:parameters {:query {:x int?, :y int?}}
         :responses {200 {:body {:total pos-int?}}}
         :handler (fn [{{{:keys [x y]} :query} :parameters}]
                    {:status 200, :body {:total (+ x y)}})}}]
      {:data {:coercion reitit.coercion.spec/coercion
              :middleware [(exception/create-exception-middleware
                             (merge
                               exception/default-handlers
                               {:reitit.coercion/request-coercion (coercion-error-handler 400)
                                :reitit.coercion/response-coercion (coercion-error-handler 500)}))
                           coercion/coerce-request-middleware
                           coercion/coerce-response-middleware]}})))

(app
  {:uri "/plus"
   :request-method :get
   :query-params {"x" "1", "y" "fail"}})
; => ...
; -- Spec failed --------------------
;
;   {:x ..., :y "fail"}
;                ^^^^^^
;
; should satisfy
;
;   int?



(app
  {:uri "/plus"
   :request-method :get
   :query-params {"x" "1", "y" "-2"}})
; => ...
;-- Spec failed --------------------
;
;   {:total -1}
;           ^^
;
; should satisfy
;
;   pos-int?

Optimizations​

The coercion middlewares are compiled against a route. In the middleware compilation step the actual coercer implementations are constructed for the defined models. Also, the middleware doesn't mount itself if a route doesn't have :coercion and :parameters or :responses defined.

We can query the compiled middleware chain for the routes:

(require '[reitit.core :as r])

(-> (ring/get-router app)
    (r/match-by-name ::plus)
    :result :post :middleware
    (->> (mapv :name)))
; [::mw/coerce-exceptions
;  ::mw/coerce-request
;  ::mw/coerce-response]

Route without coercion defined:

(app {:request-method :get, :uri "/api/ping"})
; {:status 200, :body "pong"}

Has no mounted middleware:

(-> (ring/get-router app)
    (r/match-by-name ::ping)
    :result :get :middleware
    (->> (mapv :name)))
; []