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

core/src/main/scala-3/sttp/tapir/generic/auto/SchemaMagnoliaDerivation.scala

@@ -99,6 +99,7 @@ trait SchemaMagnoliaDerivation {
                   val schemaName = subtypeNameToSchemaName(s)
                   genericDerivationConfig.toDiscriminatorValue(schemaName) -> SRef(schemaName)
                 }.toMap
+                // TODO
               baseCoproduct.addDiscriminatorField(FieldName(d), discriminatorMapping = discriminatorMapping)
             case None => baseCoproduct
           }

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

@@ -361,6 +361,16 @@ 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 {
+    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))

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/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