定义投影以映射到嵌套的案例类

我有这些案例类:

case class PolicyHolder(id : String, firstName : String, lastName : String)
case class Policy(address : Future[Address], policyHolder : Future[PolicyHolder], created : RichDateTime, duration : RichDuration )

然后我为Policy定义了一个光滑的模式

class PolicyDAO(tag: Tag) extends Table[Policy](tag, "POLICIES") with DbConfig {
  def address = column[String]("ADDRESS", O.PrimaryKey)
  def policyHolder = foreignKey("POLICY_HOLDER_FK", address, TableQuery[PolicyHolderDAO])(_.id)

  def created = column[RichDateTime]("CREATED")
  def duration = column[String]("DURATION")

  def * = (address, policyHolder, created, duration) <> (Policy.apply, Policy.unapply)
}

对我来说,正确定义此投影以将policyHolder我的Policycase类中的字段从外键值映射到case类的实际实例的最佳方法是什么PolicyHolder.