chore: Transformation.identity vs empty

akka-projection-grpc/src/main/scala/akka/projection/grpc/internal/EventProducerServiceImpl.scala

@@ -39,9 +39,10 @@ import com.google.protobuf.timestamp.Timestamp
 import io.grpc.Status
 import org.slf4j.Logger
 import org.slf4j.LoggerFactory
-
 import scala.concurrent.Future
 
+import akka.projection.grpc.producer.scaladsl.EventProducer.Transformation
+
 /**
  * INTERNAL API
  */
@@ -78,6 +79,10 @@ import scala.concurrent.Future
       eventsBySlicesPerStreamId.contains(s.streamId) ||
       eventsBySlicesStartingFromSnapshotsPerStreamId.contains(s.streamId),
       s"No events by slices query defined for stream id [${s.streamId}]")
+    require(
+      s.transformation ne Transformation.empty,
+      s"Transformation is not defined for stream id [${s.streamId}]. " +
+      "Use Transformation.identity to pass through each event as is.")
   }
 
   private val protoAnySerialization = new ProtoAnySerialization(system)

akka-projection-grpc/src/main/scala/akka/projection/grpc/producer/scaladsl/EventProducer.scala

@@ -108,7 +108,10 @@ object EventProducer {
     val empty: Transformation = new Transformation(
       mappers = Map.empty,
       orElse = envelope =>
-        Future.failed(new IllegalArgumentException(s"Missing transformation for event [${envelope.event.getClass}]")))
+        Future.failed(
+          new IllegalArgumentException(
+            s"Missing transformation for event [${envelope.event.getClass}]. " +
+            "Use Transformation.identity to pass through each event as is.")))
 
     /**
      * No transformation. Pass through each event as is.

docs/src/main/paradox/grpc.md

@@ -145,6 +145,8 @@ Java
 
 To omit an event the transformation function can return @scala[`None`]@java[`Optional.empty()`].
 
+Use  @scala[`Transformation.identity`]@java[`Transformation.identity()`] to pass through each event as is.
+
 That `EventProducer` service is started in an Akka gRPC server like this:
 
 Scala