trait HasIdAndTimestamp {
  val id: Int
  val timestamp: Long
}
case class Client(id: Int, timestamp: Long, name: String) extends HasIdAndTimestamp
case class Order(id: Int, timestamp: Long, items: List[Int], clientId: Int) extends HasIdAndTimestamp
...

case class MetaData(id: Int, timestamp: Long)
trait HasMetaData {
  val metadata: MetaData
}
case class Client(metadata: MetaData, name: String) extends HasMetaData
case class Order(metadata: MetaData, items: List[Int], clientId: Int) extends HasMetaData
...

case class MetaData(id: Int, timestamp: Long)
case class Client(name: String) extends MetaData
case class Order(items: List[Int], clientId: Int) extends MetaData
...

// type def for Metadata
object MetadataType{ // singleton wrapper, because type defs can't be toplevel
  type MetaData = (Int,Long)
}
import MetadataType.Metadata

// unpack using pattern matching
class HasMetaData(metadata: Metadata) {
  val (id,timestamp) = metadata
}
case class Client(metadata: MetaData, name: String) extends HasMetaData(metadata)
case class Order(metadata: MetaData, items: List[Int], clientId: Int) extends HasMetaData(metadata)

// alternative: even avoid constructor argument
trait HasMetaData {
  def metadata: Metadata
  lazy val (id,timestamp) = metadata // lazy to avoid init order probs
}
case class Client(metadata: MetaData, name: String) extends HasMetaData
case class Order(metadata: MetaData, items: List[Int], clientId: Int) extends HasMetaData

...
def * = ((id,timestamp), name) <> (Client.tupled, Client.unapply)
...