Slick 3.0.0: How to query one-to-many / many-to-many relations

Question

Basically the same question has been asked about a year ago for slick 2.x (scala slick one-to-many collections). I\'m wondering if there has any progression been made with t

Answer 1

IMO your code looks fine. It really depends on what feels more readable to you. Alternatively, you can use join as well:

val findBooksQuery = libraries
  .join(libraryToBooks).on(_.id === _.libraryId)
  .join(books).on(_.id === _._2.bookId)
  .result

val action = (for {
  booksResult <- findBooksQuery
} yield {
  booksResult.map { row =>
    val (libraryTableRow, libraryToBooksTableRow) = row._1
    val booksTableRow = row._2
    // TODO: Access all data from the rows and construct desired DS
  }
}

db.run(action)

You can then do a groupBy on a particular key to get the kind of data structure you are looking for. In this case, it would be more evolved as it is join across three tables. Example, add following to your query:

val findBooksQuery = libraries
  .join(libraryToBooks).on(_.id === _.libraryId)
  .join(books).on(_.id === _._2.bookId)
  // To group by libraries.id
  .groupBy(_._1.id)
  .result

Answer 2

To what you want to map to, db.run returns a Future(of something), a Future[Seq[(Library, Seq[Book])]] in your case. When mapping over a future you have access to the Seq and you can transform it to something else to get a new Future.