如何使用Slick的映射表与外键？

问题描述

我正在努力与Slick的解除嵌入和映射表。 API对我来说很奇怪，也许只是因为它的结构对我不熟悉。

我想建立一个Task / Todo-List。有两个实体：

• 任务：每个任务都有一个对下一个任务的可选引用。这样就构建了一个链表。意图是用户可以通过他的优先级来订购任务。这个顺序是由任务到任务的引用表示的。



• TaskList：表示一个带有标签和对列表第一个任务的引用的TaskList。

  
  
  

  案例类任务（id：Option [Long]，title：String，nextTask：Option [Task]）

case class TaskList（label：String，firstTask：Option [Task]）

---

现在我试图为这两个实体写一个数据访问对象（DAO）。

  import scala.slick.driver.H2Driver.simple._ 
 import slick.lifted.MappedTypeMapper 
 
隐式val会话：Session = Database.threadLocalSession 
 val queryById = Tasks.createFinderBy（t => t.id）
 
 def task（id：Long）：Option [Task] = queryById（id）.firstOption 
 
 private object任务扩展表[任务]（TASKS）{
 def id = column [Long]（ID，O.PrimaryKey，O.AutoInc）
 def title = column [String ]（TITLE）
 def nextTaskId = column [Option [Long]]（NEXT_TASK_ID）
 def nextTask = foreignKey（NEXT_TASK_FK，nextTaskId，Tasks）（_。id）
 def * = id〜title〜nextTask<> （任务，Task.unapply _）
} 
 
私有对象TaskLists extends Table [TaskList]（TASKLISTS）{
 def label = column [String]（LABEL ，O.PrimaryKey）
 def firstTaskId = column [Option [Long]]（FIRST_TASK_ID）
 def firstTask = foreignKey（FIRST_TASK_FK，firstTaskId，Tasks）（_。id）
 def * = label〜firstTask<> （Task，Task.unapply _）
} 
  

问题出现在 $ c>。

• 无法找到类型
的evidence参数的隐式值scala.slick.lifted.TypeMapper [ scala.slick.lifted.ForeignKeyQuery [SlickTaskRepository.this.Tasks.type，justf0rfun.bookmark.model.Task]]


• 可以找不到scala.slick.lifted.TypeMapper类型的证据的隐式值参数[scala.slick.lifted.ForeignKeyQuery [SlickTaskRepository.this.Tasks.type，justf0rfun.bookmark.model.Task]]

我试图用下面的TypeMapper解决，但是也不编译。

  implicit val taskMapper = MappedTypeMapper.base [Option [Long]，Option [Task]]（
 option => option match {
 case Some id）=> task（id）
 case _ => None 
}，
 option => option match {
 case Some（task）=& id 
 case _ =>无
}）
  

• 找不到参数tm的隐含值：scala.slick.lifted.TypeMapper [Option [justf0rfun.bookmark.model.Task]]


• 方法base：的参数不够（implicit tm：scala.slick.lifted.TypeMapper [Option [justf0rfun.bookmark.model.Task]]）scala.slick.lifted.BaseTypeMapper [选项[长]]。未指定的值参数tm。

主要问题：如何使用Slick提升的嵌入和映射表正确的方式？

提前感谢。

解决方案

简短的回答是：使用ids而不是对象引用，并使用Slick查询来解除引用id。

这将使你的case类看起来像这样：

  case类任务（id：Option [Long]，title：String，nextTaskId：Option [Long]）
 case class TaskList（label：String，firstTaskId： ]）
  

我会在某个时候发布关于这个主题的文章并链接到这里。 p>

I'm struggling with Slick's lifted embedding and mapped tables. The API feels strange to me, maybe just because it is structured in a way that's unfamiliar to me.

I want to build a Task/Todo-List. There are two entities:

• Task: Each task has a an optional reference to the next task. That way a linked list is build. The intention is that the user can order the tasks by his priority. This order is represented by the references from task to task.

• TaskList: Represents a TaskList with a label and a reference to the first Task of the list.

  case class Task(id: Option[Long], title: String, nextTask: Option[Task])
case class TaskList(label: String, firstTask: Option[Task])

---

Now I tried to write a data access object (DAO) for these two entities.

import scala.slick.driver.H2Driver.simple._
import slick.lifted.MappedTypeMapper

implicit val session: Session = Database.threadLocalSession
val queryById = Tasks.createFinderBy( t => t.id )

def task(id: Long): Option[Task] = queryById(id).firstOption

  private object Tasks extends Table[Task]("TASKS") {
    def id = column[Long]("ID", O.PrimaryKey, O.AutoInc)
    def title = column[String]("TITLE")
    def nextTaskId = column[Option[Long]]("NEXT_TASK_ID")
    def nextTask = foreignKey("NEXT_TASK_FK", nextTaskId, Tasks)(_.id)
    def * = id ~ title ~ nextTask <> (Task, Task.unapply _)
  }

  private object TaskLists extends Table[TaskList]("TASKLISTS") {
    def label = column[String]("LABEL", O.PrimaryKey)
    def firstTaskId = column[Option[Long]]("FIRST_TASK_ID")
    def firstTask = foreignKey("FIRST_TASK_FK", firstTaskId, Tasks)(_.id)
    def * = label ~ firstTask <> (Task, Task.unapply _)
  }

Unfortunately it does not compile. The problems are in the * projection of both tables at nextTask respective firstTask.

• could not find implicit value for evidence parameter of typescala.slick.lifted.TypeMapper[scala.slick.lifted.ForeignKeyQuery[SlickTaskRepository.this.Tasks.type,justf0rfun.bookmark.model.Task]]
• could not find implicit value for evidence parameter of type scala.slick.lifted.TypeMapper[scala.slick.lifted.ForeignKeyQuery[SlickTaskRepository.this.Tasks.type,justf0rfun.bookmark.model.Task]]

I tried to solve that with the following TypeMapper but that does not compile, too.

  implicit val taskMapper = MappedTypeMapper.base[Option[Long], Option[Task]](
    option => option match {
      case Some(id) => task(id)
      case _ => None
    },
    option => option match {
      case Some(task) => task.id
      case _ => None
    })

• could not find implicit value for parameter tm: scala.slick.lifted.TypeMapper[Option[justf0rfun.bookmark.model.Task]]
• not enough arguments for method base: (implicit tm: scala.slick.lifted.TypeMapper[Option[justf0rfun.bookmark.model.Task]])scala.slick.lifted.BaseTypeMapper[Option[Long]]. Unspecified value parameter tm.

Main question: How to use Slick's lifted embedding and mapped tables the right way? How to I get this to work?

Thanks in advance.

解决方案

The short answer is: Use ids instead of object references and use Slick queries to dereference ids. You can put the queries into methods for re-use.

That would make your case classes look like this:

case class Task(id: Option[Long], title: String, nextTaskId: Option[Long])
case class TaskList(label: String, firstTaskId: Option[Long])

I'll publish an article about this topic at some point and link it here.

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