Play框架中的编译错误

Question

我正在使用Play框架和Scala进行登录过程，我收到以下错误：

No Json serializer as JsObject found for type play.api.libs.json.JsValue. Try to implement an implicit OWrites or OFormat for this type

这是我的型号代码。我做错了什么？

def checkUser(user: Signin): Future[Option[Signin]] = {
  val query = BSONDocument("_email" -> user.email, "password" -> user.password)
  collection.flatMap(_.find(query).requireOne[Signin])
}