fix: default mapping is not being serialized with the correct shape

baywet · baywet · commit fe4a25f1f25c · 2026-07-08T08:25:00.000-04:00
diff --git a/src/Microsoft.OpenApi/Models/OpenApiDiscriminator.cs b/src/Microsoft.OpenApi/Models/OpenApiDiscriminator.cs
@@ -58,7 +58,7 @@ public void SerializeAsV32(IOpenApiWriter writer)
             if (DefaultMapping != null)
             {
                 writer.WritePropertyName("defaultMapping");
-                DefaultMapping.SerializeAsV32(writer);
+                WriteMappingReference(writer, DefaultMapping);
             }
 
             // extensions
@@ -79,7 +79,7 @@ public void SerializeAsV31(IOpenApiWriter writer)
             if (DefaultMapping != null)
             {
                 writer.WritePropertyName("x-oas-default-mapping");
-                DefaultMapping.SerializeAsV31(writer);
+                WriteMappingReference(writer, DefaultMapping);
             }
 
             // extensions
@@ -110,13 +110,19 @@ private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version
             // mapping
             writer.WriteOptionalMap(OpenApiConstants.Mapping, Mapping, (w, s) =>
             {
-                if (!string.IsNullOrEmpty(s.Reference.ReferenceV3) && s.Reference.ReferenceV3 is not null)
-                {
-                    w.WriteValue(s.Reference.ReferenceV3);
-                }
+                WriteMappingReference(w, s);
             });
         }
 
+        private static void WriteMappingReference(IOpenApiWriter writer, OpenApiSchemaReference schemaReference)
+        {
+            var reference = schemaReference.Reference.ReferenceV3;
+            if (!string.IsNullOrEmpty(reference))
+            {
+                writer.WriteValue(reference);
+            }
+        }
+
         /// <summary>
         /// Serialize <see cref="OpenApiDiscriminator"/> to Open Api v2.0
         /// </summary>
diff --git a/test/Microsoft.OpenApi.Tests/Models/OpenApiDiscriminatorTests.cs b/test/Microsoft.OpenApi.Tests/Models/OpenApiDiscriminatorTests.cs
@@ -0,0 +1,38 @@
+﻿// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System.Text.Json.Nodes;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace Microsoft.OpenApi.Tests.Models
+{
+    [Collection("DefaultSettings")]
+    public class OpenApiDiscriminatorTests
+    {
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2, "defaultMapping", "Pet", true)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2, "defaultMapping", "#/components/schemas/Pet", true)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2, "defaultMapping", "Pet", false)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1, "x-oas-default-mapping", "Pet", true)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1, "x-oas-default-mapping", "#/components/schemas/Pet", true)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1, "x-oas-default-mapping", "Pet", false)]
+        public async Task SerializeDefaultMappingAsReferenceString(OpenApiSpecVersion specVersion, string propertyName, string referenceId, bool useDocument)
+        {
+            // Arrange
+            var document = useDocument ? new OpenApiDocument() : null;
+            var discriminator = new OpenApiDiscriminator
+            {
+                PropertyName = "pet_type",
+                DefaultMapping = new OpenApiSchemaReference(referenceId, document)
+            };
+
+            // Act
+            var actual = JsonNode.Parse(await discriminator.SerializeAsJsonAsync(specVersion));
+
+            // Assert
+            Assert.NotNull(actual);
+            Assert.Equal("#/components/schemas/Pet", actual[propertyName]?.GetValue<string>());
+        }
+    }
+}
