Те не го правят лесно. Ровейки се в java, стигнах до това решение:
import org.bson.codecs.DecoderContext
import org.bson.codecs.configuration.CodecRegistries.{fromProviders, fromRegistries}
import org.bson.codecs.configuration.CodecRegistry
import org.bson.{BsonDocumentReader, BsonDocumentWrapper}
import org.mongodb.scala.bson.codecs.{DEFAULT_CODEC_REGISTRY, Macros}
import org.mongodb.scala.bson.collection.mutable.Document
import scala.reflect.classTag
case class Person(firstName: String, lastName: String)
object MongoTest extends App {
val personCodecProvider = Macros.createCodecProvider[Person]()
val codecRegistry: CodecRegistry = fromRegistries(fromProviders(personCodecProvider), DEFAULT_CODEC_REGISTRY)
val document = Document("firstName" -> "first", "lastName" -> "last")
val bsonDocument = BsonDocumentWrapper.asBsonDocument(document, DEFAULT_CODEC_REGISTRY)
val bsonReader = new BsonDocumentReader(bsonDocument)
val decoderContext = DecoderContext.builder.build
val codec = codecRegistry.get(classTag[Person].runtimeClass)
val person: Person = codec.decode(bsonReader, decoderContext).asInstanceOf[Person]
println(s"person: $person")
}