Add an encoded discriminator value attribute for coproducts, use it to render const constraints

core/src/main/scala/sttp/tapir/Schema.scala

@@ -361,6 +361,26 @@ object Schema extends LowPrioritySchema with SchemaCompanionMacros {
     val Attribute: AttributeKey[Tuple] = new AttributeKey[Tuple]("sttp.tapir.Schema.Tuple")
   }
 
+  /** For coproduct schemas, when there's a discriminator field, used to attach the encoded value of the discriminator field. Such value is
+    * added to the discriminator field schemas in each of the coproduct's subtypes. When rendering OpenAPI/JSON schema, these values are
+    * converted to `const` constraints on fields.
+    */
+  case class EncodedDiscriminatorValue(v: String)
+  object EncodedDiscriminatorValue {
+    /*
+    Implementation note: the discriminator value constraint is in fact an enum validator with a single possible enum value. Hence an
+    alternative design would be to add such validators to discriminator fields, instead of an attribute. However, this has two drawbacks:
+    1. when adding discriminator fields using `addDiscriminatorField`, we don't have access to the decoded discriminator value - only
+       to the encoded one, via reverse mapping lookup
+    2. the validator doesn't necessarily make sense, as it can't be used to validate the deserialiszd object. Usually the discriminator
+       fields don't even exist on the high-level representations.
+    That's why instead of re-using the validators, we decided to use a specialised attribute.
+     */
+
+    val Attribute: AttributeKey[EncodedDiscriminatorValue] =
+      new AttributeKey[EncodedDiscriminatorValue]("sttp.tapir.Schema.EncodedDiscriminatorValue")
+  }
+
   /** @param typeParameterShortNames
     *   full name of type parameters, name is legacy and kept only for backward compatibility
     */

core/src/main/scala/sttp/tapir/SchemaType.scala

@@ -129,11 +129,39 @@ object SchemaType {
         discriminatorSchema: Schema[D] = Schema.string,
         discriminatorMapping: Map[String, SRef[_]] = Map.empty
     ): SCoproduct[T] = {
+      // used to add encoded discriminator value attributes
+      val reverseDiscriminatorByNameMapping: Map[SName, String] = discriminatorMapping.toList.map { case (v, ref) => (ref.name, v) }.toMap
+
       SCoproduct(
         subtypes.map {
-          case s @ Schema(st: SchemaType.SProduct[Any @unchecked], _, _, _, _, _, _, _, _, _, _)
-              if st.fields.forall(_.name != discriminatorName) =>
-            s.copy(schemaType = st.copy(fields = st.fields :+ SProductField[Any, D](discriminatorName, discriminatorSchema, _ => None)))
+          case s @ Schema(st: SchemaType.SProduct[Any @unchecked], _, _, _, _, _, _, _, _, _, _) =>
+            // first, ensuring that the discriminator field is added to the schema type - it might already be present
+            var targetSt =
+              if (st.fields.forall(_.name != discriminatorName))
+                st.copy(fields = st.fields :+ SProductField[Any, D](discriminatorName, discriminatorSchema, _ => None))
+              else st
+
+            // next, modifying the discriminator field, by adding the value attribute (if a value can be found)
+            targetSt = targetSt.copy(fields = targetSt.fields.map { field =>
+              if (field.name == discriminatorName) {
+                val discriminatorValue = s.name.flatMap { subtypeName =>
+                  reverseDiscriminatorByNameMapping.get(subtypeName)
+                }
+
+                discriminatorValue match {
+                  case Some(v) =>
+                    SProductField(
+                      field.name,
+                      field.schema.attribute(Schema.EncodedDiscriminatorValue.Attribute, Schema.EncodedDiscriminatorValue(v)),
+                      field.get
+                    )
+                  case None => field
+                }
+
+              } else field
+            })
+
+            s.copy(schemaType = targetSt)
           case s => s
         },
         Some(SDiscriminator(discriminatorName, discriminatorMapping))

core/src/test/scala/sttp/tapir/SchemaMacroTest.scala

@@ -304,7 +304,14 @@ class SchemaMacroTest extends AnyFlatSpec with Matchers with TableDrivenProperty
 
     schemaType.subtypes.foreach { childSchema =>
       val childProduct = childSchema.schemaType.asInstanceOf[SProduct[_]]
-      childProduct.fields.find(_.name.name == "kind") shouldBe Some(SProductField(FieldName("kind"), Schema.string, (_: Any) => None))
+      val discValue = if (childSchema.name.get.fullName == "sttp.tapir.SchemaMacroTestData.User") "user" else "org"
+      childProduct.fields.find(_.name.name == "kind") shouldBe Some(
+        SProductField(
+          FieldName("kind"),
+          Schema.string.attribute(Schema.EncodedDiscriminatorValue.Attribute, Schema.EncodedDiscriminatorValue(discValue)),
+          (_: Any) => None
+        )
+      )
     }
   }
 

core/src/test/scala/sttp/tapir/generic/SchemaGenericAutoTest.scala

@@ -245,7 +245,13 @@ class SchemaGenericAutoTest extends AsyncFlatSpec with Matchers {
     schemaType.asInstanceOf[SCoproduct[Entity]].subtypes should contain theSameElementsAs List(
       Schema(
         SProduct[Organization](
-          List(field(FieldName("name"), Schema(SString())), field(FieldName("who_am_i"), Schema(SString())))
+          List(
+            field(FieldName("name"), Schema(SString())),
+            field(
+              FieldName("who_am_i"),
+              Schema(SString()).attribute(Schema.EncodedDiscriminatorValue.Attribute, Schema.EncodedDiscriminatorValue("Organization"))
+            )
+          )
         ),
         Some(SName("sttp.tapir.generic.Organization"))
       ),
@@ -254,15 +260,21 @@ class SchemaGenericAutoTest extends AsyncFlatSpec with Matchers {
           List(
             field(FieldName("first"), Schema(SString())),
             field(FieldName("age"), Schema(SInteger(), format = Some("int32"))),
-            field(FieldName("who_am_i"), Schema(SString()))
+            field(
+              FieldName("who_am_i"),
+              Schema(SString()).attribute(Schema.EncodedDiscriminatorValue.Attribute, Schema.EncodedDiscriminatorValue("Person"))
+            )
           )
         ),
         Some(SName("sttp.tapir.generic.Person"))
       ),
       Schema(
         SProduct[UnknownEntity.type](
           List(
-            field(FieldName("who_am_i"), Schema(SString()))
+            field(
+              FieldName("who_am_i"),
+              Schema(SString()).attribute(Schema.EncodedDiscriminatorValue.Attribute, Schema.EncodedDiscriminatorValue("UnknownEntity"))
+            )
           )
         ),
         Some(SName("sttp.tapir.generic.UnknownEntity"))

docs/apispec-docs/src/main/scala/sttp/tapir/docs/apispec/schema/TSchemaToASchema.scala

@@ -112,9 +112,13 @@ private[docs] class TSchemaToASchema(
     // The primary motivation for using schema name as fallback title is to improve Swagger UX with
     // `oneOf` schemas in OpenAPI 3.1. See https://github.com/softwaremill/tapir/issues/3447 for details.
     def fallbackTitle = tschema.name.map(fallbackSchemaTitle)
+
+    val const = tschema.attribute(TSchema.EncodedDiscriminatorValue.Attribute).map(_.v).map(v => ExampleSingleValue(v))
+
     oschema
-      .copy(title = titleFromAttr orElse fallbackTitle)
+      .copy(title = titleFromAttr.orElse(fallbackTitle))
       .copy(uniqueItems = tschema.attribute(UniqueItems.Attribute).map(_.uniqueItems))
+      .copy(const = const)
   }
 
   private def addMetadata(oschema: ASchema, tschema: TSchema[_]): ASchema = {

docs/asyncapi-docs/src/test/resources/expected_coproduct_with_discriminator.yml

@@ -0,0 +1,69 @@
+asyncapi: 2.6.0
+info:
+  title: discriminator
+  version: '1.0'
+channels:
+  /animals:
+    subscribe:
+      operationId: onAnimals
+      message:
+        $ref: '#/components/messages/Animal'
+    publish:
+      operationId: sendAnimals
+      message:
+        $ref: '#/components/messages/GetAnimal'
+    bindings:
+      ws:
+        method: GET
+components:
+  schemas:
+    GetAnimal:
+      title: GetAnimal
+      type: object
+      required:
+        - name
+      properties:
+        name:
+          type: string
+    Animal:
+      title: Animal
+      oneOf:
+        - $ref: '#/components/schemas/Cat'
+        - $ref: '#/components/schemas/Dog'
+      discriminator: pet
+    Cat:
+      title: Cat
+      type: object
+      required:
+        - name
+        - pet
+      properties:
+        name:
+          type: string
+        pet:
+          type: string
+          const: Cat
+    Dog:
+      title: Dog
+      type: object
+      required:
+        - name
+        - breed
+        - pet
+      properties:
+        name:
+          type: string
+        breed:
+          type: string
+        pet:
+          type: string
+          const: Dog
+  messages:
+    GetAnimal:
+      payload:
+        $ref: '#/components/schemas/GetAnimal'
+      contentType: application/json
+    Animal:
+      payload:
+        $ref: '#/components/schemas/Animal'
+      contentType: application/json

docs/asyncapi-docs/src/test/scala/sttp/tapir/docs/asyncapi/VerifyAsyncAPIYamlTest.scala

@@ -133,7 +133,11 @@ class VerifyAsyncAPIYamlTest extends AnyFunSuite with Matchers {
       .out(
         webSocketBody[Fruit, CodecFormat.Json, Int, CodecFormat.Json](AkkaStreams)
           // TODO: missing `RequestInfo.example(example: EndpointIO.Example)` and friends
-          .pipe(e => e.copy(requestsInfo = e.requestsInfo.example(Example.of(Fruit("apple")).name("Apple").summary("Sample representation of apple"))))
+          .pipe(e =>
+            e.copy(requestsInfo =
+              e.requestsInfo.example(Example.of(Fruit("apple")).name("Apple").summary("Sample representation of apple"))
+            )
+          )
       )
 
     val expectedYaml = loadYaml("expected_json_example_name_summary.yml")
@@ -232,6 +236,22 @@ class VerifyAsyncAPIYamlTest extends AnyFunSuite with Matchers {
     noIndentation(yaml) shouldBe loadYaml("expected_flags_header.yml")
   }
 
+  test("should work with discriminators") {
+    case class GetAnimal(name: String)
+    sealed trait Animal
+    case class Cat(name: String) extends Animal
+    case class Dog(name: String, breed: String) extends Animal
+    implicit val configuration: sttp.tapir.generic.Configuration = sttp.tapir.generic.Configuration.default.withDiscriminator("pet")
+
+    val animalEndpoint = endpoint.get
+      .in("animals")
+      .out(webSocketBody[GetAnimal, CodecFormat.Json, Animal, CodecFormat.Json](AkkaStreams))
+
+    val yaml = AsyncAPIInterpreter().toAsyncAPI(animalEndpoint, "discriminator", "1.0").toYaml
+
+    noIndentation(yaml) shouldBe loadYaml("expected_coproduct_with_discriminator.yml")
+  }
+
   private def loadYaml(fileName: String): String = {
     noIndentation(Source.fromInputStream(getClass.getResourceAsStream(s"/$fileName")).getLines().mkString("\n"))
   }

docs/openapi-docs/src/test/resources/coproduct/expected_coproduct_discriminator.yml

@@ -33,6 +33,7 @@ components:
       properties:
         name:
           type: string
+          const: sml
     Person:
       title: Person
       type: object
@@ -42,6 +43,7 @@ components:
       properties:
         name:
           type: string
+          const: john
         age:
           type: integer
           format: int32

docs/openapi-docs/src/test/resources/coproduct/expected_coproduct_discriminator_nested.yml

@@ -41,6 +41,7 @@ components:
       properties:
         name:
           type: string
+          const: sml
     Person:
       title: Person
       type: object
@@ -50,6 +51,7 @@ components:
       properties:
         name:
           type: string
+          const: john
         age:
           type: integer
           format: int32

docs/openapi-docs/src/test/resources/coproduct/expected_coproduct_discriminator_with_enum_circe.yml

@@ -37,3 +37,4 @@ components:
             - red
         shapeType:
           type: string
+          const: Square

docs/openapi-docs/src/test/resources/coproduct/expected_coproduct_independent.yml

@@ -74,6 +74,7 @@ components:
           type: string
         kind:
           type: string
+          const: organization
     Person:
       title: Person
       type: object
@@ -100,4 +101,5 @@ components:
           type: integer
           format: int32
         kind:
-          type: string
+          type: string
+          const: person