Use the extension method from JsonSchema.NET to get a dicriminator object from the schema and serialize it

Author: MaggieKimani1

src/Microsoft.OpenApi.Hidi/Formatters/PowerShellFormatter.cs

@@ -348,9 +348,9 @@ private static JsonSchema CopySchema(JsonSchema schema, JsonSchema newSchema)
             {
                 schemaBuilder.MinProperties(minProperties);
             }
-            if (schema.GetDiscriminator() == null && newSchema.GetOpenApiDiscriminator() is { } discriminator)
+            if (schema.GetDiscriminator() == null && newSchema.GetDiscriminator() is { } discriminator)
             {
-                schemaBuilder.Discriminator(discriminator);
+                schemaBuilder.Discriminator(discriminator.PropertyName, discriminator.Mapping, discriminator.Extensions);
             }
             if (schema.GetOpenApiExternalDocs() == null && newSchema.GetOpenApiExternalDocs() is { } externalDocs)
             {

src/Microsoft.OpenApi/Validations/Rules/JsonSchemaRules.cs

@@ -84,9 +84,9 @@ public static class JsonSchemaRules
                     // discriminator
                     context.Enter("discriminator");
 
-                    if (jsonSchema.GetRef() != null && jsonSchema.GetOpenApiDiscriminator() != null)
+                    if (jsonSchema.GetRef() != null && jsonSchema.GetDiscriminator() != null)
                     {
-                        var discriminatorName = jsonSchema.GetOpenApiDiscriminator()?.PropertyName;
+                        var discriminatorName = jsonSchema.GetDiscriminator()?.PropertyName;
 
                         if (!ValidateChildSchemaAgainstDiscriminator(jsonSchema, discriminatorName))
                         {

src/Microsoft.OpenApi/Writers/OpenApiWriterBase.cs

@@ -574,7 +574,25 @@ public void WriteJsonSchemaWithoutReference(IOpenApiWriter writer, JsonSchema sc
             writer.WriteProperty(OpenApiConstants.Nullable, schema.GetNullable(), false);
 
             // discriminator
-            writer.WriteOptionalObject(OpenApiConstants.Discriminator, schema.GetOpenApiDiscriminator(), (w, d) => d.SerializeAsV3(w));
+            var discriminator = schema.GetDiscriminator();
+            if (discriminator != null)
+            {                
+                writer.WriteStartObject();
+                
+                // propertyName
+                writer.WriteProperty(OpenApiConstants.PropertyName, discriminator.PropertyName);
+
+                // mapping
+                writer.WriteOptionalMap(OpenApiConstants.Mapping, (IDictionary<string, string>)discriminator.Mapping, (w, s) => w.WriteValue(s));
+
+                if (version == OpenApiSpecVersion.OpenApi3_1 && discriminator.Extensions.Any())
+                {
+                    // extensions
+                    writer.WriteExtensions((IDictionary<string, Interfaces.IOpenApiExtension>)discriminator.Extensions, OpenApiSpecVersion.OpenApi3_1);
+                }
+
+                writer.WriteEndObject();
+            }
 
             // readOnly
             writer.WriteProperty(OpenApiConstants.ReadOnly, schema.GetReadOnly(), false);