Scala Slick implicit conversion of multiple types in raw SQL query

问题

I'm working on a Scala Play Framework 2.2 project, and I'm using play-slick 0.5.0.8 as my DB access layer.

I have the following code in one of my DAO objects:

def randomByBlahAndDate(blah: Blah, newerThan: LocalDate)(implicit s: Session): Option[Photo] = {
  sql"select * from photos where blah = $blah and imgDate > $newerThan order by rand()".as[Photo].headOption
}

As you can see, it does some tricky stuff like ordering by rand(), which is why I decided to go the raw SQL route. In any case, I get the following error on compilation:

could not find implicit value for parameter pconv: scala.slick.jdbc.SetParameter[(models.Blah.Blah, org.joda.time.LocalDate)]

It appears as though slick is trying to convert both of my types together, as a set... strange. I have an implicit type converter on my Blah enumeration, which works properly when inserting and fetching Photo objects:

def enumToStringMapper(enum: Enumeration) = MappedTypeMapper.base[enum.Value, String](
enum   => enum.toString,
string => enum.withName(string))

implicit val FormatMapper = enumToStringMapper(Blah)

I also import com.github.tototoshi.slick.JodaSupport._ to support the LocalDate conversion. That also works just fine when inserting and fetching Photo objects.

Any ideas? Perhaps some sort of better query mechanism to support what I need (enum equality, date comparison, and rand() ordering)? Thanks.

UPDATE: 2013-10-27

I am now trying to do the following, with no luck:

def recordGuess(date: LocalDate, correctBlah: Blah, incorrectBlah: Blah, isCorrect: Boolean)(implicit s: Session) {
  val correctIncrement = if(isCorrect) 1L else 0L
  sqlu"insert into stats (date, correctBlah, incorrectBlah, impressions, guesses, correct) values ($date, $correctBlah, $incorrectBlah, 1, 1, $correctIncrement) on duplicate key update guesses = guesses + 1, correct = correct + $correctIncrement".first
}

Which again, doesn't work:

could not find implicit value for parameter pconv: scala.slick.jdbc.SetParameter[(org.joda.time.LocalDate, models.Blah.Blah, models.Blah.Blah)]

However, this time I see no good, easy way around it. Seems like Typesafe's sql and sqlu in slick don't support implicit conversions.

回答1:

I haven't been able to find a solution to the implicit conversion problem, but I've found a workaround using more traditional slick syntax, with scala.util.Random.shuffle:

def randomByBlahAndDate(blah: Blah, newerThan: LocalDate)(implicit s: Session): Option[Photo] = {
  val photos = Query(Photos).where(_.imgDate > newerThan).where(_.blah === blah).run
  val r = new scala.util.Random(scala.compat.Platform.currentTime)
  r.shuffle(photos).headOption
}

I'm uncertain about the efficiency compared to using MySQL's rand(), but this will work for the time being.

来源：https://stackoverflow.com/questions/19535120/scala-slick-implicit-conversion-of-multiple-types-in-raw-sql-query