Generic CRUD operations using Slick 2.0

问题

I am trying to write a generic CRUD trait for Slick 2.0. The trait should a) provide generic methods to read/update/delete entities as well as b) abstract from the database. Following this slick example (database abstraction) and this article (CRUD trait) I came up with the following (shortened) code snippet:

trait Profile {
  val profile: JdbcProfile
}

trait Crud[T <: AbstractTable[A], A] { this: Profile =>
  import profile.simple._

  val qry: TableQuery[T]

  def countAll()(implicit session: Session): Int = {
    qry.length.run
  }

  def getAll()(implicit session: Session): List[A] = {
      qry.list // <-- type mismatch; found: List[T#TableElementType] required: List[A]
  }
}

The code is invalid due to a type mismatch. The return type of the 2nd function seems to be of type List[T#TableElementType] but needs to be List[A]. Any ideas on how to solve the issue. Additional references to further readings on generic Slick 2.0 operations are welcome too.

回答1:

type TableElementType is abstract inside of class AbstractTable[A]. Scala doesn't know about any relationship between A and TableElementType. class Table on the other hand defines final type TableElementType = A, which tells Scala about this relationship (and apparently Scala is smart enough to use the final annotation to know that the relationanship even holds for a subtype T <: Table[A] eventhough Table[A] is not co-variant in A).

So you need to use T <: Table[A] instead of T <: AbstractTable[A]. And because Table is inside the Slick driver cake (as in cake pattern), you need to move your Crud into your cake as well. Cakes are viral.

trait Profile {
  val profile: JdbcProfile
}
trait CrudComponent{ this: Profile =>
  import profile.simple._

  trait Crud[T <: Table[A], A] {

    val qry: TableQuery[T]

    def countAll()(implicit session: Session): Int = {
      qry.length.run
    }

    def getAll()(implicit session: Session): List[A] = {
        qry.list // <-- works as intended
    }
  }
}

来源：https://stackoverflow.com/questions/22338041/generic-crud-operations-using-slick-2-0