How to do MongoDB Group aggregation with Scala MongoDB driver

MongoDB Scala Group By.sc

import org.mongodb.scala.bson.collection.mutable.Document
import org.mongodb.scala.{MongoClient, MongoCollection, MongoDatabase}
import org.mongodb.scala.model.Aggregates._
import org.mongodb.scala.model.Accumulators._
import com.profuzio.MongoHelper._

val mongoClient: MongoClient = MongoClient.apply("mongodb://localhost")
val database: MongoDatabase = mongoClient.getDatabase("kiwanda")
val collection: MongoCollection[Document] = database.getCollection("labor")

val v = collection.aggregate(Seq(
  group(
    // This is supposed to be a TExpression.  Not sure what it is but this works 
    Document( "rvp" -> "$rvp", "rm" -> "$rm", "am" -> "$am" ),    
    sum("totalhours", "$TotalHours"))
)).results()

println(v)