使用Slick 2.0的通用CRUD操作

问题描述

我正在尝试为Slick 2.0编写通用的CRUD特性.特质应该 a)提供通用方法来读取/更新/删除实体以及 b)从数据库中提取.以下这个精巧的示例(数据库抽象)和本文(CRUD特性)，我想出了以下(缩短的)代码片段:

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]
  }
}

由于类型不匹配，代码无效.第二个函数的返回类型似乎为List[T#TableElementType]类型，但必须为List [A].关于如何解决问题的任何想法.也欢迎您提供有关Slick 2.0通用操作的进一步阅读的其他参考.

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.

推荐答案

type TableElementType是class AbstractTable[A]内部的抽象. Scala不知道A和TableElementType之间的任何关系.另一方面，class Table定义了final type TableElementType = A，它向Scala告知了这种关系(显然，Scala足够聪明，可以使用final注释来知道即使Table[A]是在A中不是协变的.)

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).

因此，您需要使用T <: Table[A]而不是T <: AbstractTable[A].并且由于Table位于Slick驱动程序蛋糕中(如蛋糕模式一样)，因此您也需要将Crud移入蛋糕中.蛋糕是病毒.

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
    }
  }
}

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