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POHow to use Slick's mapped tables with foreign keys?
 

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  1. POHow to use Slick's mapped tables with foreign keys?
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    <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. </p> <p>I want to build a Task/Todo-List. There are two entities: </p> <ul> <li>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.</li> <li><p>TaskList: Represents a TaskList with a label and a reference to the first Task of the list.</p> <p><code>case class Task(id: Option[Long], title: String, nextTask: Option[Task])</code><br> <code>case class TaskList(label: String, firstTask: Option[Task])</code></p></li> </ul> <hr> <p>Now I tried to write a data access object (DAO) for these two entities.</p> <pre><code>import scala.slick.driver.H2Driver.simple._ import slick.lifted.MappedTypeMapper implicit val session: Session = Database.threadLocalSession val queryById = Tasks.createFinderBy( t =&gt; 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 &lt;&gt; (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 &lt;&gt; (Task, Task.unapply _) } </code></pre> <p>Unfortunately it does not compile. The problems are in the <code>*</code> projection of both tables at <code>nextTask</code> respective <code>firstTask</code>. </p> <ul> <li><code>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]]</code></li> <li><code>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]]</code></li> </ul> <p>I tried to solve that with the following TypeMapper but that does not compile, too.</p> <pre><code> implicit val taskMapper = MappedTypeMapper.base[Option[Long], Option[Task]]( option =&gt; option match { case Some(id) =&gt; task(id) case _ =&gt; None }, option =&gt; option match { case Some(task) =&gt; task.id case _ =&gt; None }) </code></pre> <ul> <li><code>could not find implicit value for parameter tm: scala.slick.lifted.TypeMapper[Option[justf0rfun.bookmark.model.Task]]</code></li> <li><code>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.</code></li> </ul> <p><strong>Main question: How to use Slick's lifted embedding and mapped tables the right way? How to I get this to work?</strong></p> <p>Thanks in advance.</p>
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