From f26dc1ab19e1356861b8bdcfd243e2eb412bca20 Mon Sep 17 00:00:00 2001
From: Joel Kaasinen <joel.kaasinen@metosin.fi>
Date: Fri, 10 Oct 2025 08:48:40 +0300
Subject: [PATCH] feat: :default-values-for-optional-keys for malli coercion

---
 doc/coercion/malli_coercion.md                |  9 ++++++--
 .../src/reitit/coercion/malli.cljc            |  9 ++++----
 test/cljc/reitit/coercion_test.cljc           | 21 +++++++++++++++++++
 3 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/doc/coercion/malli_coercion.md b/doc/coercion/malli_coercion.md
index c7ecc34d..eb25b27c 100644
--- a/doc/coercion/malli_coercion.md
+++ b/doc/coercion/malli_coercion.md
@@ -73,9 +73,10 @@ Using `create` with options to create the coercion instead of `coercion`:
   {:transformers {:body {:default reitit.coercion.malli/default-transformer-provider
                          :formats {"application/json" reitit.coercion.malli/json-transformer-provider}}
                   :string {:default reitit.coercion.malli/string-transformer-provider}
-                  :response {:default reitit.coercion.malli/default-transformer-provider}}
+                  :response {:default reitit.coercion.malli/default-transformer-provider
+                             :formats {"application/json" reitit.coercion.malli/json-transformer-provider}}}
    ;; set of keys to include in error messages
-   :error-keys #{:type :coercion :in :schema :value :errors :humanized #_:transformed}
+   :error-keys #{:type :coercion :in #_:schema :value #_:errors :humanized #_:transformed}
    ;; support lite syntax?
    :lite true
    ;; schema identity function (default: close all map schemas)
@@ -88,6 +89,10 @@ Using `create` with options to create the coercion instead of `coercion`:
    :strip-extra-keys true
    ;; add/set default values
    :default-values true
+   ;; add/set defaults also for optional keys. Corresponds to :malli.transform/add-optional-keys
+   :default-values-for-optional-keys false
+   ;; encode-error
+   :encode-error nil
    ;; malli options
    :options nil})
 ```
diff --git a/modules/reitit-malli/src/reitit/coercion/malli.cljc b/modules/reitit-malli/src/reitit/coercion/malli.cljc
index 813bbc20..568f605d 100644
--- a/modules/reitit-malli/src/reitit/coercion/malli.cljc
+++ b/modules/reitit-malli/src/reitit/coercion/malli.cljc
@@ -9,8 +9,7 @@
             [malli.swagger :as swagger]
             [malli.transform :as mt]
             [malli.util :as mu]
-            [reitit.coercion :as coercion]
-            [clojure.string :as string]))
+            [reitit.coercion :as coercion]))
 
 ;;
 ;; coercion
@@ -27,11 +26,11 @@
 
 (defn- -provider [transformer]
   (reify TransformationProvider
-    (-transformer [_ {:keys [strip-extra-keys default-values]}]
+    (-transformer [_ {:keys [strip-extra-keys default-values default-values-for-optional-keys]}]
       (mt/transformer
        (if strip-extra-keys (mt/strip-extra-keys-transformer))
        transformer
-       (if default-values (mt/default-value-transformer))))))
+       (if default-values (mt/default-value-transformer {:malli.transform/add-optional-keys default-values-for-optional-keys}))))))
 
 (def string-transformer-provider (-provider (mt/string-transformer)))
 (def json-transformer-provider (-provider (mt/json-transformer)))
@@ -118,6 +117,8 @@
    :strip-extra-keys true
    ;; add/set default values
    :default-values true
+   ;; add/set defaults also for optional keys. Corresponds to :malli.transform/add-optional-keys
+   :default-values-for-optional-keys false
    ;; encode-error
    :encode-error nil
    ;; malli options
diff --git a/test/cljc/reitit/coercion_test.cljc b/test/cljc/reitit/coercion_test.cljc
index 0fcfd125..e1f0cd20 100644
--- a/test/cljc/reitit/coercion_test.cljc
+++ b/test/cljc/reitit/coercion_test.cljc
@@ -140,6 +140,27 @@
         (let [m (r/match-by-path r "/none/kikka/abba")]
           (is (= nil (coercion/coerce! m))))))))
 
+(deftest malli-query-parameter-coercion-test
+  (let [router (fn [coercion]
+                 (r/router ["/test"
+                            {:coercion coercion
+                             :parameters {:query [:map
+                                                  [:a [:string {:default "a"}]]
+                                                  [:x {:optional true} [:keyword {:default :a}]]]}}]
+                           {:compile coercion/compile-request-coercers}))]
+    (testing "default values for :optional query keys do not get added"
+      (is (= {:query {:a "a"}}
+             (-> (r/match-by-path (router reitit.coercion.malli/coercion) "/test")
+                 (assoc :query-params {})
+                 (coercion/coerce!)))))
+    (testing "default values for :optional query keys get added when :default-values-for-optional-keys is set"
+      (is (= {:query {:a "a" :x :a}}
+             (-> (r/match-by-path (router (reitit.coercion.malli/create
+                                           (assoc reitit.coercion.malli/default-options
+                                                  :default-values-for-optional-keys true))) "/test")
+                 (assoc :query-params {})
+                 (coercion/coerce!)))))))
+
 (defn match-by-path-and-coerce! [router path]
   (if-let [match (r/match-by-path router path)]
     (assoc match :parameters (coercion/coerce! match))))