scala slick query return value

Question

I am rather new at Scala and have been struggling with slick and can't see how to return the results of a query to the calling method

I have a simple UserDto

case class UserDto(val firstName:String,
  val lastName:String,
  val userName:String,
  val isAdmin:Boolean) {}

a User table object

object User extends Table[(String, String, String, Boolean)]("USER") {

  def firstName = column[String]("FIRST_NAME")
  def lastName = column[String]("LAST_NAME")
  def userName = column[String]("USER_NAME")
  def admin = column[Boolean]("IS_ADMIN")

  def * = firstName ~ lastName ~ userName ~ admin

}

and a query class

class UserQuerySlickImpl(dataSource:DataSource) {

  def getResults(userName:String):Option[UserDto] = {
    var resultDto:Option[UserDto] = None

    Database.forDataSource(dataSource) withSession {
      val q = for {u <- User if u.userName is userName}
      yield (u.firstName, u.lastName, u.userName, u.admin)

      for (t <- q) {
        t match {
          case (f:String, l:String, u:String, a:Boolean) => 
            resultDto = Some(new UserDto(f, l, u, a))
        }
      }
    }
    resultDto
  }
}

I can query the database and get the user that matches the username, but the only way I could figure out how to return that user is by creating a var outside of the Database.forDataSource....{}.

Is there a better way that does not use the var but returns the resultDto directly.

also is there a way to construct the UserDto directly from the first for comprehension rather than needing the second for (t <- q) ...

I am using slick_2.10.0-M7, version 0.11.1.

Answer 1

Your q is a query, not a list of results. The presence of foreach might be a little confusing in that respect, but to obtain a List of results, you need to do q.list first. That gives you methods like map and foldLeft and so on.

If you want to get a single/the first result in an Option, use q.firstOption. Once you have that, you can map your function over the resulting 'Option[(...)]` to transform the tuple into the desired DTO.

An alternative way would be to specify a custom mapping that automatically maps your result tuples to some case class by using the <> operator, see http://slick.typesafe.com/doc/0.11.2/lifted-embedding.html#tables:

case class User(id: Option[Int], first: String, last: String) 

object Users extends Table[User]("users") {
  def id = column[Int]("id", O.PrimaryKey, O.AutoInc)
  def first = column[String]("first")
  def last = column[String]("last")
  def * = id.? ~ first ~ last <> (User, User.unapply _)
}

Answer 2

I haven't toyed with Slick yet but if it's reasonable (by which I mean consistent with Scala conventions) you should be able to do something like

def getResults(userName:String):Option[UserDto] =
  Database.forDataSource(dataSource) withSession {
    val q = for {u <- User if u.userName is userName}
      yield (u.firstName, u.lastName, u.userName, u.admin)

    q.firstOption map { case (f, l, u, a) => UserDto(f, l, u, a) }
  }

This is exactly what you would do if q was a List[(String, String, String, Boolean)].

Cleaning this up a bit, you can write

def getResults(userName:String):Option[UserDto] =
  Database.forDataSource(dataSource) withSession {
    (for (u <- User if u.userName is userName)
      yield UserDto(u.firstName, u.lastName, u.userName, u.admin)).firstOption
  }

Otherwise, you should be able to use

q foreach { 
   case (f, l, u, a) => return Some(UserDto(f, l, u, a))
}
return None

Generally, return statements like this one should be avoided, so hopefully q's type gives you something more functional to work with.