Slick 3: how to drop and take on collections with some relations - scala

Question

I'm working with Play! Scala 2.4 and Slick 3.
I have a many to many relations as following:
class Artists(tag: Tag) extends Table[Artist](tag, "artists") {
def id = column[Long]("artistid", O.PrimaryKey, O.AutoInc)
def name = column[String]("name")
def * = (id.?, name) <> ((Artist.apply _).tupled, Artist.unapply)
}
The relation table:
class ArtistsGenres(tag: Tag) extends Table[ArtistGenreRelation](tag, "artistsgenres") {
def artistId = column[Long]("artistid")
def genreId = column[Int]("genreid")
def * = (artistId, genreId) <> ((ArtistGenreRelation.apply _).tupled, ArtistGenreRelation.unapply)
def aFK = foreignKey("artistid", artistId, artists)(_.id, onDelete = ForeignKeyAction.Cascade)
def bFK = foreignKey("genreid", genreId, genres)(_.id, onDelete = ForeignKeyAction.Cascade)
}
and the third table:
class Genres(tag: Tag) extends Table[Genre](tag, "genres") {
def id = column[Int]("genreid", O.PrimaryKey, O.AutoInc)
def name = column[String]("name")
def * = (id.?, name) <> ((Genre.apply _).tupled, Genre.unapply)
}
Until now I just wanted to get all the artists by their genre names as following (and their genres as well):
def findAllByGenre(genreName: String, offset: Int, numberToReturn: Int): Future[Seq[ArtistWithGenre]] = {
val query = for {
genre <- genres if genre.name === genreName
artistGenre <- artistsGenres if artistGenre.genreId === genre.id
artist <- artists joinLeft
(artistsGenres join genres on (_.genreId === _.id)) on (_.id === _._1.artistId)
if artist._1.id === artistGenre.artistId
} yield artist
db.run(query.result) map { seqArtistAndOptionalGenre =>
ArtistsAndOptionalGenresToArtistsWithGenres(seqArtistAndOptionalGenre)
}
}
The method ArtistsAndOptionalGenresToArtistsWithGenres groups the response by artists. This worked like a charm. Now I want to limit the number of artists I get from the database.
But I don't manage to use correctly the slick functions take and drop: indeed as my query returns a list of artists and relations, If I add a take before the .result I don't receive the number of artists I want to get (depending of the number of relations the artists have).
I could drop and take after that I have grouped my result by artist, but I see a problem here: the SGBDR won't optimize the request, i.e. I will get all the artists (it can be a lot), proceed the groupBy and after take a bit instead of limit the number of artist returned before the groupBy.

Answer 1

I found the following solution (with 2 queries but 1 DB call):
def findAllByGenre(genreName: String, offset: Int, numberToReturn: Int): Future[Seq[ArtistWithWeightedGenres]] = {
val query = for {
genre <- genres.filter(_.name === genreName)
artistGenre <- artistsGenres.filter(_.genreId === genre.id)
artist <- artists.filter(_.id === artistGenre.artistId)
} yield artist
val artistsIdFromDB = query.drop(offset).take(numberToReturn) map (_.id)
val query2 = for {
artistWithGenres <- artists.filter(_.id in artistsIdFromDB) joinLeft
(artistsGenres join genres on (_.genreId === _.id)) on (_.id === _._1.artistId)
} yield artistWithGenres
db.run(query2.result) map { seqArtistAndOptionalGenre =>
ArtistsAndOptionalGenresToArtistsWithGenres(seqArtistAndOptionalGenre)
} map(_.toVector)
}
If anyone has a better solution...