From e56ec4d60d822c987c6648a39fac84e971d8dfb1 Mon Sep 17 00:00:00 2001
From: souvikghosh04 <souvikofficial04@gmail.com>
Date: Mon, 21 Sep 2026 17:07:24 +0530
Subject: [PATCH] Fix #3704: return 200 empty payload for cached reads with
 empty result

When entity/runtime caching is enabled and a read returns zero rows, the cached
value surfaces as a default(JsonElement) with JsonValueKind.Undefined and no
backing document. ParseResultIntoJsonDocument called
JsonSerializer.SerializeToUtf8Bytes on it, which throws InvalidOperationException
via JsonElement.WriteTo -> CheckValidInstance(), producing HTTP 500.

Guard the Undefined case and return null, matching the non-cached path that
renders an empty payload. This path is shared by REST and GraphQL. Adds a
regression unit test.
---
 src/Core/Resolvers/SqlQueryEngine.cs          |  8 ++++++++
 .../UnitTests/SqlQueryEngineHelperTests.cs    | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/Core/Resolvers/SqlQueryEngine.cs b/src/Core/Resolvers/SqlQueryEngine.cs
index f567251771..a17f8bc952 100644
--- a/src/Core/Resolvers/SqlQueryEngine.cs
+++ b/src/Core/Resolvers/SqlQueryEngine.cs
@@ -443,6 +443,14 @@ public object ResolveList(JsonElement array, ObjectField fieldSchema, ref IMetad
 
         private static JsonDocument? ParseResultIntoJsonDocument(JsonElement? result)
         {
+            // An empty result set surfaces as a default (JsonValueKind.Undefined) JsonElement with no
+            // backing document; serializing it throws InvalidOperationException. Return null to match the
+            // non-cached path, which renders an empty payload (e.g. {"value":[]}).
+            if (result is { ValueKind: JsonValueKind.Undefined })
+            {
+                return null;
+            }
+
             byte[] jsonBytes = JsonSerializer.SerializeToUtf8Bytes(result);
             return JsonDocument.Parse(jsonBytes);
         }
diff --git a/src/Service.Tests/UnitTests/SqlQueryEngineHelperTests.cs b/src/Service.Tests/UnitTests/SqlQueryEngineHelperTests.cs
index e12b59d6cf..793b6993cc 100644
--- a/src/Service.Tests/UnitTests/SqlQueryEngineHelperTests.cs
+++ b/src/Service.Tests/UnitTests/SqlQueryEngineHelperTests.cs
@@ -46,6 +46,25 @@ public void ParseResultIntoJsonDocument_HandlesValuesAndNull(string? json, bool
             Assert.AreEqual(hasObject ? JsonValueKind.Object : JsonValueKind.Null, result.RootElement.ValueKind);
         }
 
+        /// <summary>
+        /// Regression test for https://github.com/Azure/data-api-builder/issues/3704
+        /// An empty cached read surfaces as a default (JsonValueKind.Undefined) JsonElement that has no
+        /// backing document. Serializing it previously threw InvalidOperationException (HTTP 500).
+        /// ParseResultIntoJsonDocument must instead return null, matching the non-cached empty-result path.
+        /// </summary>
+        [TestMethod]
+        public void ParseResultIntoJsonDocument_UndefinedElement_ReturnsNull()
+        {
+            JsonElement? undefined = default(JsonElement);
+            MethodInfo method = typeof(SqlQueryEngine).GetMethod(
+                "ParseResultIntoJsonDocument",
+                BindingFlags.Static | BindingFlags.NonPublic)!;
+
+            JsonDocument? result = (JsonDocument?)method.Invoke(null, new object?[] { undefined });
+
+            Assert.IsNull(result);
+        }
+
         /// <summary>
         /// Verifies stored-procedure execution returns the first result object and maps empty or absent result arrays to null.
         /// </summary>