Add union type redirect

core/src/main/scala-2/caliban/schema/SchemaDerivation.scala

@@ -22,9 +22,16 @@ trait SchemaDerivation[R] extends LowPriorityDerivedSchema {
 
   type Typeclass[T] = Schema[R, T]
 
+  def isValueType[T](ctx: ReadOnlyCaseClass[Typeclass, T]): Boolean =
+    ctx.annotations.exists {
+      case GQLValueType() => true
+      case _              => false
+    }
+
   def combine[T](ctx: ReadOnlyCaseClass[Typeclass, T]): Typeclass[T] = new Typeclass[T] {
     override def toType(isInput: Boolean, isSubscription: Boolean): __Type =
-      if (ctx.isValueClass && ctx.parameters.nonEmpty) ctx.parameters.head.typeclass.toType_(isInput, isSubscription)
+      if ((ctx.isValueClass || isValueType(ctx)) && ctx.parameters.nonEmpty)
+        ctx.parameters.head.typeclass.toType_(isInput, isSubscription)
       else if (isInput)
         makeInputObject(
           Some(ctx.annotations.collectFirst { case GQLInputName(suffix) => suffix }
@@ -70,7 +77,7 @@ trait SchemaDerivation[R] extends LowPriorityDerivedSchema {
 
     override def resolve(value: T): Step[R] =
       if (ctx.isObject) PureStep(EnumValue(getName(ctx)))
-      else if (ctx.isValueClass && ctx.parameters.nonEmpty) {
+      else if ((ctx.isValueClass || isValueType(ctx)) && ctx.parameters.nonEmpty) {
         val head = ctx.parameters.head
         head.typeclass.resolve(head.dereference(value))
       } else {

core/src/main/scala-3/caliban/schema/SchemaDerivation.scala

@@ -105,13 +105,14 @@ trait SchemaDerivation[R] {
           }
         }
       case m: Mirror.ProductOf[A] =>
+        lazy val annotations = Macros.annotations[A]
         lazy val fields = recurse[m.MirroredElemLabels, m.MirroredElemTypes]()
         lazy val info = Macros.typeInfo[A]
-        lazy val annotations = Macros.annotations[A]
         lazy val paramAnnotations = Macros.paramAnnotations[A].toMap
         new Schema[R, A] {
           def toType(isInput: Boolean, isSubscription: Boolean): __Type =
-            if (isInput)
+            if (isValueType(annotations) && fields.nonEmpty) fields.head._3.toType_(isInput, isSubscription)
+            else if (isInput)
               makeInputObject(
                 Some(annotations.collectFirst { case GQLInputName(suffix) => suffix }
                   .getOrElse(customizeInputTypeName(getName(annotations, info)))),
@@ -156,7 +157,10 @@ trait SchemaDerivation[R] {
 
           def resolve(value: A): Step[R] =
             if (fields.isEmpty) PureStep(EnumValue(getName(annotations, info)))
-            else {
+            else if (isValueType(annotations) && fields.nonEmpty) {
+              val head = fields.head
+              head._3.resolve(value.asInstanceOf[Product].productElement(head._4))
+            } else {
               val fieldsBuilder = Map.newBuilder[String, Step[R]]
               fields.foreach { case (label, _, schema, index) =>
                 val fieldAnnotations = paramAnnotations.getOrElse(label, Nil)
@@ -165,7 +169,7 @@ trait SchemaDerivation[R] {
               ObjectStep(getName(annotations, info), fieldsBuilder.result())
             }
         }
-    }
+  }
 
   // see https://github.com/graphql/graphql-spec/issues/568
   private def fixEmptyUnionObject(t: __Type): __Type =
@@ -197,6 +201,12 @@ trait SchemaDerivation[R] {
       }
     }
 
+  private def isValueType(annotations: Seq[Any]): Boolean =
+    annotations.exists {
+      case GQLValueType() => true
+      case _ => false
+    }
+
   private def getName(annotations: Seq[Any], label: String): String =
     annotations.collectFirst { case GQLName(name) => name }.getOrElse(label)
 

core/src/main/scala/caliban/schema/Annotations.scala

@@ -41,4 +41,9 @@ object Annotations {
    * Annotation to make a sealed trait a union instead of an enum
    */
   case class GQLUnion() extends StaticAnnotation
+
+  /**
+   * Annotation to make a union or interface redirect to a value type
+   */
+  case class GQLValueType() extends StaticAnnotation
 }

core/src/test/scala/caliban/execution/ExecutionSpec.scala

@@ -9,7 +9,7 @@ import caliban.TestUtils._
 import caliban.Value.{ BooleanValue, IntValue, StringValue }
 import caliban.introspection.adt.__Type
 import caliban.parsing.adt.LocationInfo
-import caliban.schema.Annotations.{ GQLInterface, GQLName }
+import caliban.schema.Annotations.{ GQLInterface, GQLName, GQLValueType }
 import caliban.schema.{ ArgBuilder, Schema, Step, Types }
 import zio.{ IO, Task, UIO, ZIO }
 import zio.stream.ZStream
@@ -918,6 +918,40 @@ object ExecutionSpec extends DefaultRunnableSpec {
             """{"foos":[{"id":123,"bar":{"id":234}}]}"""
           )
         )
+      },
+      testM("union redirect") {
+        sealed trait Foo
+
+        case class Bar(int: Int, common: Boolean) extends Foo
+
+        case class Baz(value: String, common: Boolean)
+
+        @GQLValueType
+        case class Redirect(baz: Baz) extends Foo
+
+        case class Queries(foos: List[Foo])
+
+        val queries = Queries(
+          List(
+            Bar(42, common = true),
+            Redirect(Baz("hello", common = false))
+          )
+        )
+
+        val api: GraphQL[Any] = GraphQL.graphQL(RootResolver(queries))
+        val interpreter       = api.interpreter
+        val query             = gqldoc("""{
+            foos {
+              ... on Bar { common int }
+              ... on Baz { common value }
+            }
+            }""")
+
+        assertM(interpreter.flatMap(_.execute(query)).map(_.data.toString))(
+          equalTo(
+            """{"foos":[{"common":true,"int":42},{"common":false,"value":"hello"}]}"""
+          )
+        )
       }
     )
 }

core/src/test/scala/caliban/schema/SchemaSpec.scala

@@ -1,8 +1,10 @@
 package caliban.schema
 
+import caliban.Rendering
+
 import java.util.UUID
 import caliban.introspection.adt.{ __DeprecatedArgs, __Type, __TypeKind }
-import caliban.schema.Annotations.{ GQLInterface, GQLUnion }
+import caliban.schema.Annotations.{ GQLInterface, GQLUnion, GQLValueType }
 import zio.blocking.Blocking
 import zio.console.Console
 import zio.query.ZQuery
@@ -126,6 +128,24 @@ object SchemaSpec extends DefaultRunnableSpec {
         implicit val somethingSchema: Schema[Any, Something] = Schema.gen[Something].rename("SomethingElse")
 
         assert(Types.innerType(introspectSubscription[Something]).name)(isSome(equalTo("SomethingElse")))
+      },
+      test("union redirect") {
+        case class Queries(union: RedirectingUnion)
+
+        implicit val queriesSchema: Schema[Any, Queries] = Schema.gen[Queries]
+
+        val types      = Types.collectTypes(introspect[Queries])
+        val subTypes   = types.find(_.name.contains("RedirectingUnion")).flatMap(_.possibleTypes)
+        val fieldNames =
+          subTypes.toList.flatMap(_.flatMap(_.fields(__DeprecatedArgs()).map(_.map(_.name)))).toSet.flatten
+        assert(subTypes.map(_.flatMap(_.name)))(
+          isSome(
+            hasSameElements(
+              List("A", "B")
+            )
+          )
+        ) &&
+        assert(fieldNames)(hasSameElements(List("common")))
       }
     )
 
@@ -148,6 +168,18 @@ object SchemaSpec extends DefaultRunnableSpec {
     case object B extends EnumLikeUnion
   }
 
+  @GQLUnion
+  sealed trait RedirectingUnion
+
+  object RedirectingUnion {
+    case class B(common: Int)
+
+    case class A(common: Int) extends RedirectingUnion
+
+    @GQLValueType
+    case class Redirect(value: B) extends RedirectingUnion
+  }
+
   @GQLInterface
   sealed trait EnumLikeInterface
   object EnumLikeInterface {

vuepress/docs/docs/schema.md

@@ -122,6 +122,40 @@ type Mechanic {
 }
 ```
 
+If your type needs to be shared between multiple unions you can use the `@GQLValueType` annotation to have caliban
+proxy to another type beyond the sealed trait.
+
+```scala
+case class Pilot(callSign: String)
+
+sealed trait Role
+object Role {
+  case class Captain(shipName: String) extends Role
+  case class Engineer(specialty: String) extends Role
+
+  @GQLValueType
+  case class Proxy(pilot: Pilot)
+}
+```
+
+This will produce the following GraphQL Types:
+
+```graphql
+union Role = Captain | Engineer | Pilot
+
+type Captain {
+  shipName: String!
+}
+
+type Engineer {
+  specialty: String!
+}
+
+type Pilot {
+  callSign: String!
+}
+```
+
 If you prefer an `Interface` instead of a `Union` type, add the `@GQLInterface` annotation to your sealed trait.
 An interface will be created with all the fields that are common to the case classes extending the sealed trait, as long as they return the same type.
 
@@ -181,6 +215,7 @@ Caliban supports a few annotations to enrich data types:
 - `@GQLDeprecated("reason")` allows deprecating a field or an enum value.
 - `@GQLInterface` to force a sealed trait generating an interface instead of a union.
 - `@GQLDirective(directive: Directive)` to add a directive to a field or type.
+- `@GQLValueType` forces a type to behave as a value type for derivation. Meaning that caliban will ignore the outer type and take the first case class parameter as the real type.
 
 ## Java 8 Time types