Scala for comprehension for Slick Query

Question

I'm new to Scala and I'm building a Play app to learn Scala and Slick. The app has todo lists and each list has items. Each user has multiple lists. I've written a controller to get the list as JSON, and the JSON includes all the items in the list.

  def viewList(id: Long) = AuthenticatedAction.async { implicit request =>
    val userId = request.user.toLong;
    db.run(todoLists.filter(listById(id, userId)).result.headOption).flatMap { list =>
      list match {
        case Some(_) => {
          val json = toJson(list).as[JsObject]
          // Fetch the items in this list and add them to the JSON response
          db.run(todoItems.filter(_.listId === id).sortBy(_.text).result).map { items => 
            Ok(json + ("items" -> toJson(items)))
          }
        }
        case None => Future.successful(NotFound)
      }
    }
  }

Is it possible to write this function using a for comprehension? I have a nested flatMap+map call, so it seems like it should be possible.

every call to map, flatMap, filter can be rewritten as for comprehension, but do you really need it? This can be more verbose — ka4eli
That's an excellent question - I'm new to Scala so I don't really know. I've been told that for comprehension can make things easier to read. — Nathan
if you understand the code with higher order functions, I think you don't. May be just for practising or for someone else better understanding, imho — ka4eli

Denis Borovikov Denis Borovikov · Accepted Answer · 2015-08-10T13:09:04

Yes, it is possible. Like this:

def viewList(id: Long) = AuthenticatedAction.async { implicit request =>
  val userId = request.user.toLong
  for {
    list <- db.run(todoLists.filter(listById(id, userId)).result.headOption)
    resp <- list.fold[Future[Result]](Future.successful(NotFound)) { _ =>
      val json = toJson(list).as[JsObject]
      // Fetch the items in this list and add them to the JSON response
      db.run(todoItems.filter(_.listId === id).sortBy(_.text).result).map { items => 
        Ok(json + ("items" -> toJson(items)))
      }
    }
  } yield resp 
}

Another variant:

  def viewList(id: Long) = AuthenticatedAction.async { implicit request =>
    val userId = request.user.toLong
    for {
      list <- db.run(todoLists.filter(listById(id, userId)).result.headOption)
      items <- db.run(todoItems.filter(_.listId === id).sortBy(_.text).result)
    } yield list match {
      case Some(_) => Ok(toJson(list).as[JsObject] + ("items" -> toJson(items)))
      case None => NotFound
    }
  }

Scala for comprehension for Slick Query

1 Answers