From 043b1f97eef01f026f7842919f5aa85e8a58a11c Mon Sep 17 00:00:00 2001
From: Laurence Isla <lau.isla.c@gmail.com>
Date: Fri, 29 Aug 2025 16:21:32 -0500
Subject: [PATCH] fix: empty enum in 'preferParams' openAPI parameter

---
 CHANGELOG.md                             | 1 +
 src/PostgREST/Response/OpenAPI.hs        | 3 ++-
 test/spec/Feature/OpenApi/OpenApiSpec.hs | 7 +++++++
 3 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 59102f8c2..a83bf4473 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fix logging the Haskell type instead of the listener error message directly by @laurenceisla in #3588
 - Fix format of `IPv6` address logged at PostgREST startup by @taimoorzaeem in #4291
+- Fix empty enum in `preferParams` OpenAPI parameter by @laurenceisla in #4292
 
 ### Changed
 
diff --git a/src/PostgREST/Response/OpenAPI.hs b/src/PostgREST/Response/OpenAPI.hs
index 9ca526932..880106a8c 100644
--- a/src/PostgREST/Response/OpenAPI.hs
+++ b/src/PostgREST/Response/OpenAPI.hs
@@ -171,8 +171,9 @@ makePreferParam ts =
   & schema .~ ParamOther ((mempty :: ParamOtherSchema)
     & in_ .~ ParamHeader
     & type_ ?~ SwaggerString
-    & enum_ .~ JSON.decode (JSON.encode $ foldl (<>) [] (val <$> ts)))
+    & enum_ .~ if null enu then Nothing else JSON.decode (JSON.encode enu))
   where
+    enu = foldl (<>) [] (val <$> ts)
     val :: Text -> [Text]
     val = \case
       "count"      -> ["count=none"]
diff --git a/test/spec/Feature/OpenApi/OpenApiSpec.hs b/test/spec/Feature/OpenApi/OpenApiSpec.hs
index c035bbe7b..f9d26fafe 100644
--- a/test/spec/Feature/OpenApi/OpenApiSpec.hs
+++ b/test/spec/Feature/OpenApi/OpenApiSpec.hs
@@ -1085,6 +1085,13 @@ spec = describe "OpenAPI" $ do
         immutableGet `shouldNotBe` Nothing
         immutablePost `shouldNotBe` Nothing
 
+    it "does not include empty enum in the preferParams parameter" $ do
+      r <- simpleBody <$> get "/"
+      let preferParams = r ^? key "parameters" . key "preferParams" . key "enum"
+
+      liftIO $ do
+        preferParams `shouldBe` Nothing
+
   describe "Security" $
     it "does not include security or security definitions by default" $ do
       r <- simpleBody <$> get "/"