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

[adamw]

An alternative approach to #3763

cc @kamilkloch @zorba128 @seveneves

Closes #3909

[kamilkloch]

Coproduct may be also rendererd without a discriminator field (Json object nesting). We need to make sure this case is covered as well.

[adamw]

Yes, this applies only to discriminators. I think if there's no discriminator, then we don't have to do anything, that is we shouldn't add any constraints?

[adamw]

You can see how this is rendered in the OpenAPI yamls in the tests - let me know if that's what you were after

[kamilkloch]

Thank you, will do!

[seveneves]

@adamw thank you!

I will test this with my setup

[zorba128]

Please take a look at test that illustrates issue with async api:

class VerifyAsyncAPIYamlTest extends AnyFunSuite with Matchers {
  // ...
 
  /** TODO: fixing this requires custom rendering of ASchema (no discriminator mapping, just the discriminator field name).
   * It seems best place to fix it is at `sttp.apispec.asyncapi.circe` - injecting custom `discriminator` encoder for ASchema.
   * 
   * Current output:
   * ```
   * Animal:
   *   title: Animal
   *   oneOf:
   *   - $ref: '#/components/schemas/Cat'
   *   - $ref: '#/components/schemas/Dog'
   *   discriminator:
   *     propertyName: pet
   *     mapping:
   *       Cat: '#/components/schemas/Cat'
   *       Dog: '#/components/schemas/Dog'
   * ```
   * Desired:
   * ```
   * Animal:
   *   title: Animal
   *   oneOf:
   *   - $ref: '#/components/schemas/Cat'
   *   - $ref: '#/components/schemas/Dog'
   *   discriminator: pet
   * ```
   */
  test("should render coproduct using discriminator correctly") {
    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))
        // .responsesExamples(List(Cat("Tom"), Dog("Spike", "Bulldog")))

    val yaml = AsyncAPIInterpreter().toAsyncAPI(animalEndpoint, "discriminator mapping", "1.0").toYaml
    println(yaml)

    noIndentation(yaml) shouldBe loadYaml("expected_coproduct_with_discriminator.yml")
  }
}

[zorba128]

See softwaremill/sttp-apispec#178

[adamw]

@zorba128 I've added your test, take a look

[zorba128]

Yay :)

[kamilkloch]

Looks good, thank you @adamw @zorba128 !