Use BaseTypeInfo API to fix up polymorphic schemas (dotnet#56908)

* Use BaseTypeInfo API to fix up polymorphic schemas

* Change schema for polymorphic types with non-abstract base class

* Add comments, update API name, and add more tests

* More feedback and tests

src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs

@@ -334,17 +334,27 @@ static bool SupportsNullableProperty(BindingSource bindingSource) =>bindingSourc
     }
 
     /// <summary>
-    /// Applies the polymorphism options to the target schema following OpenAPI v3's conventions.
+    /// Applies the polymorphism options defined by System.Text.Json to the target schema following OpenAPI v3's
+    /// conventions for the discriminator property.
     /// </summary>
     /// <param name="schema">The <see cref="JsonNode"/> produced by the underlying schema generator.</param>
     /// <param name="context">The <see cref="JsonSchemaExporterContext"/> associated with the current type.</param>
     /// <param name="createSchemaReferenceId">A delegate that generates the reference ID to create for a type.</param>
-    internal static void ApplyPolymorphismOptions(this JsonNode schema, JsonSchemaExporterContext context, Func<JsonTypeInfo, string?> createSchemaReferenceId)
+    internal static void MapPolymorphismOptionsToDiscriminator(this JsonNode schema, JsonSchemaExporterContext context, Func<JsonTypeInfo, string?> createSchemaReferenceId)
     {
-        // The `context.Path.Length == 0` check is used to ensure that we only apply the polymorphism options
+        // The `context.BaseTypeInfo == null` check is used to ensure that we only apply the polymorphism options
         // to the top-level schema and not to any nested schemas that are generated.
-        if (context.TypeInfo.PolymorphismOptions is { } polymorphismOptions && context.Path.Length == 0)
+        if (context.TypeInfo.PolymorphismOptions is { } polymorphismOptions && context.BaseTypeInfo == null)
         {
+            // System.Text.Json supports serializing to a non-abstract base class if no discriminator is provided.
+            // OpenAPI requires that all polymorphic sub-schemas have an associated discriminator. If the base type
+            // doesn't declare itself as its own derived type via [JsonDerived], then it can't have a discriminator,
+            // which OpenAPI requires. In that case, we exit early to avoid mapping the polymorphism options
+            // to the `discriminator` property and return an un-discriminated `anyOf` schema instead.
+            if (IsNonAbstractTypeWithoutDerivedTypeReference(context))
+            {
+                return;
+            }
             var mappings = new JsonObject();
             foreach (var derivedType in polymorphismOptions.DerivedTypes)
             {
@@ -376,6 +386,24 @@ internal static void ApplySchemaReferenceId(this JsonNode schema, JsonSchemaExpo
         {
             schema[OpenApiConstants.SchemaId] = schemaReferenceId;
         }
+        // If the type is a non-abstract base class that is not one of the derived types then mark it as a base schema.
+        if (context.BaseTypeInfo == context.TypeInfo &&
+            IsNonAbstractTypeWithoutDerivedTypeReference(context))
+        {
+            schema[OpenApiConstants.SchemaId] = "Base";
+        }
+    }
+
+    /// <summary>
+    /// Returns <langword ref="true" /> if the current type is a non-abstract base class that is not defined as its
+    /// own derived type.
+    /// </summary>
+    /// <param name="context">The <see cref="JsonSchemaExporterContext"/> associated with the current type.</param>
+    private static bool IsNonAbstractTypeWithoutDerivedTypeReference(JsonSchemaExporterContext context)
+    {
+        return !context.TypeInfo.Type.IsAbstract
+            && context.TypeInfo.PolymorphismOptions is { } polymorphismOptions
+            && !polymorphismOptions.DerivedTypes.Any(type => type.DerivedType == context.TypeInfo.Type);
     }
 
     /// <summary>

src/OpenApi/src/Schemas/OpenApiJsonSchema.Helpers.cs

@@ -300,7 +300,10 @@ public static void ReadProperty(ref Utf8JsonReader reader, string propertyName,
             case OpenApiSchemaKeywords.DiscriminatorMappingKeyword:
                 reader.Read();
                 var mappings = ReadDictionary<string>(ref reader);
-                schema.Discriminator.Mapping = mappings;
+                if (mappings is not null)
+                {
+                    schema.Discriminator.Mapping = mappings;
+                }
                 break;
             case OpenApiConstants.SchemaId:
                 reader.Read();

src/OpenApi/src/Schemas/OpenApiSchemaKeywords.cs

@@ -24,6 +24,5 @@ internal class OpenApiSchemaKeywords
     public const string MinItemsKeyword = "minItems";
     public const string MaxItemsKeyword = "maxItems";
     public const string RefKeyword = "$ref";
-    public const string SchemaIdKeyword = "x-schema-id";
     public const string ConstKeyword = "const";
 }

src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs

@@ -93,7 +93,7 @@ internal sealed class OpenApiSchemaService(
             var createSchemaReferenceId = optionsMonitor.Get(documentName).CreateSchemaReferenceId;
             schema.ApplyPrimitiveTypesAndFormats(context, createSchemaReferenceId);
             schema.ApplySchemaReferenceId(context, createSchemaReferenceId);
-            schema.ApplyPolymorphismOptions(context, createSchemaReferenceId);
+            schema.MapPolymorphismOptionsToDiscriminator(context, createSchemaReferenceId);
             if (context.PropertyInfo is { } jsonPropertyInfo)
             {
                 // Short-circuit STJ's handling of nested properties, which uses a reference to the

src/OpenApi/src/Services/Schemas/OpenApiSchemaStore.cs

@@ -84,6 +84,7 @@ public void PopulateSchemaIntoReferenceCache(OpenApiSchema schema, bool captureS
         // Only capture top-level schemas by ref. Nested schemas will follow the
         // reference by duplicate rules.
         AddOrUpdateSchemaByReference(schema, captureSchemaByRef: captureSchemaByRef);
+        AddOrUpdateAnyOfSubSchemaByReference(schema);
         if (schema.AdditionalProperties is not null)
         {
             AddOrUpdateSchemaByReference(schema.AdditionalProperties);
@@ -99,24 +100,56 @@ public void PopulateSchemaIntoReferenceCache(OpenApiSchema schema, bool captureS
                 AddOrUpdateSchemaByReference(allOfSchema);
             }
         }
+        if (schema.Properties is not null)
+        {
+            foreach (var property in schema.Properties.Values)
+            {
+                AddOrUpdateSchemaByReference(property);
+            }
+        }
+    }
+
+    private void AddOrUpdateAnyOfSubSchemaByReference(OpenApiSchema schema)
+    {
         if (schema.AnyOf is not null)
         {
             // AnyOf schemas in a polymorphic type should contain a reference to the parent schema
             // ID to support disambiguating between a derived type on its own and a derived type
             // as part of a polymorphic schema.
-            var baseTypeSchemaId = schema.Annotations is not null && schema.Annotations.TryGetValue(OpenApiConstants.SchemaId, out var schemaId) ? schemaId?.ToString() : null;
+            var baseTypeSchemaId = schema.Annotations is not null && schema.Annotations.TryGetValue(OpenApiConstants.SchemaId, out var schemaId)
+                ? schemaId?.ToString()
+                : null;
             foreach (var anyOfSchema in schema.AnyOf)
             {
                 AddOrUpdateSchemaByReference(anyOfSchema, baseTypeSchemaId);
             }
         }
-        if (schema.Properties is not null)
+
+        if (schema.Items is not null)
+        {
+            AddOrUpdateAnyOfSubSchemaByReference(schema.Items);
+        }
+
+        if (schema.Properties is { Count: > 0 })
         {
             foreach (var property in schema.Properties.Values)
             {
-                AddOrUpdateSchemaByReference(property);
+                AddOrUpdateAnyOfSubSchemaByReference(property);
+            }
+        }
+
+        if (schema.AllOf is not null)
+        {
+            foreach (var allOfSchema in schema.AllOf)
+            {
+                AddOrUpdateAnyOfSubSchemaByReference(allOfSchema);
             }
         }
+
+        if (schema.AdditionalProperties is not null)
+        {
+            AddOrUpdateAnyOfSubSchemaByReference(schema.AdditionalProperties);
+        }
     }
 
     private void AddOrUpdateSchemaByReference(OpenApiSchema schema, string? baseTypeSchemaId = null, bool captureSchemaByRef = false)

src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.PolymorphicSchemas.cs

@@ -129,14 +129,53 @@ await VerifyOpenApiDocument(builder, document =>
     }
 
     [Fact]
-    public async Task HandlesPolymorphicTypesWithNonAbstractBaseClass()
+    public async Task HandlesPolymorphicTypesWithNonAbstractBaseClassWithNoDiscriminator()
     {
         // Arrange
         var builder = CreateBuilder();
 
         // Act
         builder.MapPost("/api", (Color color) => { });
 
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[OperationType.Post];
+            Assert.NotNull(operation.RequestBody);
+            var requestBody = operation.RequestBody.Content;
+            Assert.True(requestBody.TryGetValue("application/json", out var mediaType));
+            var schema = mediaType.Schema.GetEffective(document);
+            // Assert discriminator mappings are not configured for this type since we
+            // can't meet OpenAPI's restrictions that derived types _always_ have a discriminator
+            // property associated with them.
+            Assert.Null(schema.Discriminator);
+            Assert.Collection(schema.AnyOf,
+                schema => Assert.Equal("ColorPaintColor", schema.Reference.Id),
+                schema => Assert.Equal("ColorFabricColor", schema.Reference.Id),
+                schema => Assert.Equal("ColorBase", schema.Reference.Id));
+            // Assert schema with discriminator = "paint" has been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("ColorPaintColor", out var paintSchema));
+            Assert.Contains("$type", paintSchema.Properties.Keys);
+            Assert.Equal("paint", ((OpenApiString)paintSchema.Properties["$type"].Enum.First()).Value);
+            // Assert schema with discriminator = "fabric" has been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("ColorFabricColor", out var fabricSchema));
+            Assert.Contains("$type", fabricSchema.Properties.Keys);
+            Assert.Equal("fabric", ((OpenApiString)fabricSchema.Properties["$type"].Enum.First()).Value);
+            // Assert that schema for `Color` has been inserted into the components without a discriminator
+            Assert.True(document.Components.Schemas.TryGetValue("ColorBase", out var colorSchema));
+            Assert.DoesNotContain("$type", colorSchema.Properties.Keys);
+        });
+    }
+
+    [Fact]
+    public async Task HandlesPolymorphicTypesWithNonAbstractBaseClassAndDiscriminator()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapPost("/api", (Pet pet) => { });
+
         // Assert
         await VerifyOpenApiDocument(builder, document =>
         {
@@ -148,30 +187,113 @@ await VerifyOpenApiDocument(builder, document =>
             // Assert discriminator mappings have been configured correctly
             Assert.Equal("$type", schema.Discriminator.PropertyName);
             Assert.Collection(schema.Discriminator.Mapping,
-                item => Assert.Equal("paint", item.Key),
-                item => Assert.Equal("fabric", item.Key)
+                item => Assert.Equal("cat", item.Key),
+                item => Assert.Equal("dog", item.Key),
+                item => Assert.Equal("pet", item.Key)
             );
             Assert.Collection(schema.Discriminator.Mapping,
-                item => Assert.Equal("#/components/schemas/ColorPaintColor", item.Value),
-                item => Assert.Equal("#/components/schemas/ColorFabricColor", item.Value)
+                item => Assert.Equal("#/components/schemas/PetCat", item.Value),
+                item => Assert.Equal("#/components/schemas/PetDog", item.Value),
+                item => Assert.Equal("#/components/schemas/PetPet", item.Value)
             );
-            // Note that our implementation diverges from the OpenAPI specification here. OpenAPI
-            // requires that derived types in a polymorphic schema _always_ have a discriminator
+            // OpenAPI requires that derived types in a polymorphic schema _always_ have a discriminator
             // property associated with them. STJ permits the discriminator to be omitted from the
             // if the base type is a non-abstract class and falls back to serializing to this base
-            // type. This is a known limitation of the current implementation.
+            // type. In this scenario, we check that the base class is not included in the `anyOf`
+            // schema.
             Assert.Collection(schema.AnyOf,
-                schema => Assert.Equal("ColorPaintColor", schema.Reference.Id),
-                schema => Assert.Equal("ColorFabricColor", schema.Reference.Id),
-                schema => Assert.Equal("ColorColor", schema.Reference.Id));
-            // Assert schema with discriminator = "paint" has been inserted into the components
-            Assert.True(document.Components.Schemas.TryGetValue("ColorPaintColor", out var paintSchema));
-            Assert.Contains(schema.Discriminator.PropertyName, paintSchema.Properties.Keys);
-            Assert.Equal("paint", ((OpenApiString)paintSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
-            // Assert schema with discriminator = "fabric" has been inserted into the components
-            Assert.True(document.Components.Schemas.TryGetValue("ColorFabricColor", out var fabricSchema));
-            Assert.Contains(schema.Discriminator.PropertyName, fabricSchema.Properties.Keys);
-            Assert.Equal("fabric", ((OpenApiString)fabricSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+                schema => Assert.Equal("PetCat", schema.Reference.Id),
+                schema => Assert.Equal("PetDog", schema.Reference.Id),
+                schema => Assert.Equal("PetPet", schema.Reference.Id));
+            // Assert schema with discriminator = "dog" has been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("PetDog", out var dogSchema));
+            Assert.Contains(schema.Discriminator.PropertyName, dogSchema.Properties.Keys);
+            Assert.Equal("dog", ((OpenApiString)dogSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+            // Assert schema with discriminator = "cat" has been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("PetCat", out var catSchema));
+            Assert.Contains(schema.Discriminator.PropertyName, catSchema.Properties.Keys);
+            Assert.Equal("cat", ((OpenApiString)catSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+            // Assert schema with discriminator = "cat" has been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("PetPet", out var petSchema));
+            Assert.Contains(schema.Discriminator.PropertyName, petSchema.Properties.Keys);
+            Assert.Equal("pet", ((OpenApiString)petSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+        });
+    }
+
+    [Fact]
+    public async Task HandlesPolymorphicTypesWithNoExplicitDiscriminators()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapPost("/api", (Organism color) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[OperationType.Post];
+            Assert.NotNull(operation.RequestBody);
+            var requestBody = operation.RequestBody.Content;
+            Assert.True(requestBody.TryGetValue("application/json", out var mediaType));
+            var schema = mediaType.Schema.GetEffective(document);
+            // Assert discriminator mappings are not configured for this type since we
+            // can't meet OpenAPI's restrictions that derived types _always_ have a discriminator
+            // property associated with them.
+            Assert.Null(schema.Discriminator);
+            Assert.Collection(schema.AnyOf,
+                schema => Assert.Equal("OrganismAnimal", schema.Reference.Id),
+                schema => Assert.Equal("OrganismPlant", schema.Reference.Id),
+                schema => Assert.Equal("OrganismBase", schema.Reference.Id));
+            // Assert that schemas without discriminators have been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("OrganismAnimal", out var animalSchema));
+            Assert.DoesNotContain("$type", animalSchema.Properties.Keys);
+            Assert.True(document.Components.Schemas.TryGetValue("OrganismPlant", out var plantSchema));
+            Assert.DoesNotContain("$type", plantSchema.Properties.Keys);
+            Assert.True(document.Components.Schemas.TryGetValue("OrganismBase", out var baseSchema));
+            Assert.DoesNotContain("$type", baseSchema.Properties.Keys);
+        });
+    }
+
+    [Fact]
+    public async Task HandlesPolymorphicTypesWithSelfReference()
+    {
+        // Arrange
+        var builder = CreateBuilder();
+
+        // Act
+        builder.MapPost("/api", (Employee color) => { });
+
+        // Assert
+        await VerifyOpenApiDocument(builder, document =>
+        {
+            var operation = document.Paths["/api"].Operations[OperationType.Post];
+            Assert.NotNull(operation.RequestBody);
+            var requestBody = operation.RequestBody.Content;
+            Assert.True(requestBody.TryGetValue("application/json", out var mediaType));
+            Assert.Equal("Employee", mediaType.Schema.Reference.Id);
+            var schema = mediaType.Schema.GetEffective(document);
+            // Assert that discriminator mappings are configured correctly for type.
+            Assert.Equal("$type", schema.Discriminator.PropertyName);
+            Assert.Collection(schema.Discriminator.Mapping,
+                item => Assert.Equal("manager", item.Key),
+                item => Assert.Equal("employee", item.Key)
+            );
+            Assert.Collection(schema.Discriminator.Mapping,
+                item => Assert.Equal("#/components/schemas/EmployeeManager", item.Value),
+                item => Assert.Equal("#/components/schemas/EmployeeEmployee", item.Value)
+            );
+            // Assert that anyOf schemas use the correct reference IDs.
+            Assert.Collection(schema.AnyOf,
+                schema => Assert.Equal("EmployeeManager", schema.Reference.Id),
+                schema => Assert.Equal("EmployeeEmployee", schema.Reference.Id));
+            // Assert that schemas without discriminators have been inserted into the components
+            Assert.True(document.Components.Schemas.TryGetValue("EmployeeManager", out var managerSchema));
+            Assert.Equal("manager", ((OpenApiString)managerSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+            Assert.True(document.Components.Schemas.TryGetValue("EmployeeEmployee", out var employeeSchema));
+            Assert.Equal("employee", ((OpenApiString)employeeSchema.Properties[schema.Discriminator.PropertyName].Enum.First()).Value);
+            // Assert that the schema has a correct self-reference to the base-type. This points to the schema that contains the discriminator.
+            Assert.Equal("Employee", employeeSchema.Properties["manager"].Reference.Id);
         });
     }
 }