mahesh mahesh - 6 months ago 29
Scala Question

Scala Play Future Interdependancy

I am using play-slick for my scala Play! dummy rest API.

So, I have to fetch records from multiple tables. But, they are interdependent, i.e.

Table_1 Table_2
id1 id2

To fetch a record from Table_2 I have to fetch a record from Table_1 and then using id2 fetch from Table_2.

My controller:

def getEntity(id : Long) = Action.async {
table1DAO.findById(id) map { t1 =>
t1 map { t1Entity =>
table2DAO.findById(t1Entity.id2) map { t2 =>
t2 map { t2Entity =>
Ok(Json.toJson(CombiningClass(t1Entity, t2Entity)))
} getOrElse { Ok(Json.toJson(t1Entity)) }
} getOrElse { NoContent }

After compilation, I get:

[error] DummyController.scala:17: overloaded method value async with alternatives:
[error] [A](bodyParser: play.api.mvc.BodyParser[A])(block: play.api.mvc.Request[A] => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[A] <and>
[error] (block: play.api.mvc.Request[play.api.mvc.AnyContent] => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[play.api.mvc.AnyContent] <and>
[error] (block: => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[play.api.mvc.AnyContent]
[error] cannot be applied to (scala.concurrent.Future[Object])
[error] def getEntity(id : Long) = Action.async {
[error] ^
[error] one error found

Here's my DAO method:

def findById(id : Long): Future[Option[A]] = { === id).result.headOption)

PS: I'm very new to functional paradigm and scala, so if you can, pardon my ignorance.


To simply solve your problem:

def getEntity(id : Long) = Action.async {
  findById(id) flatMap {
    case Some(t1Entity) =>
      findById(t1Entity.id2) map { t2Opt =>
        t2Opt map { t2Entity =>
          Ok(Json.toJson(t1Entity, t2Entity))
        } getOrElse { Ok(Json.toJson(t1Entity)) }
    case None => Future.successful(NoContent)

The problem here is that you can't flatMap Option and Future together in scala. Here is a fantastic and simple article concerning this topic (with the custom FutureO monad implementation as a solution). Long story short, I would use the cats library (or even scalaz library) and OptionT feature. I slightly simplified your code.

def getEntity(id : Long) = Action.async {
  (for {
    t1 <- daoFindById(id)
    t2 <- daoFindById(t1.id2)
  } yield (t1, t2)).map{
    result => Ok(Json.toJson(result))

case class T(id2: Long)
def daoFindById(id : Long): OptionT[Future, T] = {
  OptionT[Future, T]( === id).result.headOption))

You can now easily flatMap over this OptionT monad, and don't care if you are dealing with Option or Future (for comprehension in scala is only a syntactic sugar).