fix/default mapping serialization

src/Microsoft.OpenApi/Models/OpenApiDiscriminator.cs

@@ -58,7 +58,7 @@
             if (DefaultMapping != null)
             {
                 writer.WritePropertyName("defaultMapping");
-                DefaultMapping.SerializeAsV32(writer);
+                WriteMappingReference(writer, DefaultMapping);
             }
 
             // extensions
@@ -79,7 +79,7 @@
             if (DefaultMapping != null)
             {
                 writer.WritePropertyName("x-oas-default-mapping");
-                DefaultMapping.SerializeAsV31(writer);
+                WriteMappingReference(writer, DefaultMapping);
             }
 
             // extensions
@@ -98,7 +98,7 @@
             writer.WriteEndObject();
         }
 
         private void SerializeInternal(IOpenApiWriter writer, OpenApiSpecVersion version)
         {
             Utils.CheckArgumentNull(writer);
 
@@ -108,13 +108,16 @@
             writer.WriteProperty(OpenApiConstants.PropertyName, PropertyName);
 
             // mapping
-            writer.WriteOptionalMap(OpenApiConstants.Mapping, Mapping, (w, s) =>
+            writer.WriteOptionalMap(OpenApiConstants.Mapping, Mapping, WriteMappingReference);
+        }
+
+        private static void WriteMappingReference(IOpenApiWriter writer, OpenApiSchemaReference schemaReference)
+        {
+            var reference = schemaReference.Reference.ReferenceV3;
+            if (!string.IsNullOrEmpty(reference))
             {
-                if (!string.IsNullOrEmpty(s.Reference.ReferenceV3) && s.Reference.ReferenceV3 is not null)
-                {
-                    w.WriteValue(s.Reference.ReferenceV3);
-                }
-            });
+                writer.WriteValue(reference!);
+            }
         }
 
         /// <summary>

test/Microsoft.OpenApi.Tests/Models/OpenApiDiscriminatorTests.cs

@@ -0,0 +1,75 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System.Collections.Generic;
+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>());
+        }
+
+        [Theory]
+        [InlineData(OpenApiSpecVersion.OpenApi3_0)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_1)]
+        [InlineData(OpenApiSpecVersion.OpenApi3_2)]
+        public async Task SerializeMappingValuesAsReferenceStrings(OpenApiSpecVersion specVersion)
+        {
+            // Arrange
+            var document = new OpenApiDocument();
+            var discriminator = new OpenApiDiscriminator
+            {
+                PropertyName = "pet_type",
+                Mapping = new Dictionary<string, OpenApiSchemaReference>
+                {
+                    ["simple"] = new OpenApiSchemaReference("Pet", document),
+                    ["explicit"] = new OpenApiSchemaReference("#/components/schemas/Pet", document),
+                    ["noDocument"] = new OpenApiSchemaReference("Pet")
+                }
+            };
+
+            // Act
+            var actual = JsonNode.Parse(await discriminator.SerializeAsJsonAsync(specVersion));
+
+            // Assert
+            Assert.NotNull(actual);
+            var mapping = Assert.IsAssignableFrom<JsonObject>(actual["mapping"]);
+            AssertMappingValue(mapping, "simple");
+            AssertMappingValue(mapping, "explicit");
+            AssertMappingValue(mapping, "noDocument");
+        }
+
+        private static void AssertMappingValue(JsonObject mapping, string key)
+        {
+            var value = Assert.IsAssignableFrom<JsonValue>(mapping[key]);
+            Assert.Equal("#/components/schemas/Pet", value.GetValue<string>());
+        }
+    }
+}