Scala 光滑模式leftJoin生成笛卡尔积

我有两张桌子：

    select * from "DEPARTMENTS";
 ID | NAME  | MANAGER_ID 
----+-------+------------
  1 | FOO   |          1
  3 | XXX   |          2
  4 | dept1 |           
(3 rows)

select * from "EMPLOYEES";
 NAME | LAST |      EMAIL       | PHONE | SKYPE | DEPT_ID | ID 
------+------+------------------+-------+-------+---------+----
 AAA  | xxxx | abcdef@gmail.com |       |       |         |  1
 BBB  | yyyy |                  |       |       |         |  2
(2 rows)

如果经理总共有3行，我想得到所有有经理名字的部门

但是，在我的模式中，当我这样做时：

import scala.slick.driver.PostgresDriver.simple._
import play.api.Play.current
case class DepartmentManager(id:Int,name:String,manager:String="")
case class Department(id:Option[Int],name:String,managerId:Option[Int])
class Departments (tag: Tag) extends Table[Department](tag, "DEPARTMENTS") {
  val employees = TableQuery[Employees]
  def id = column[Int]("ID", O.PrimaryKey, O.AutoInc)
  def name = column[String]("NAME", O.NotNull)
  def managerId = column[Int]("MANAGER_ID", O.Nullable)
  def manager = foreignKey("EMP_FK",managerId,employees)(_.id)
  def * = (id.?,name,managerId.?) <> (Department.tupled, Department.unapply)
}

object Departments{
  val db = play.api.db.slick.DB
  val departments = TableQuery[Departments]
//this is the problematic query
  def allWithMngr = db.withSession { implicit session =>
    val employees = TableQuery[Employees]
    val dm = (departments leftJoin employees ).list
    dm.map(x =>DepartmentManager(x._1.id.getOrElse(0),x._1.name, x._2.name + " " + x._2.last))
  }
}

结果出现错误：

[SlickException:读取结果集列路径的NULL值NULL s2._4]

这是我的员工模型：

case class Employee(name: String,last: String,email:Option[String]=None,phone:Option[String]=None,skype:Option[String]=None,department: Option[Int] = None, id: Option[Int] = None)
class Employees (tag: Tag) extends Table[Employee](tag, "EMPLOYEES") {
  def id = column[Int]("ID", O.PrimaryKey, O.AutoInc)
  def name = column[String]("NAME", O.NotNull)
  def last = column[String]("LAST", O.NotNull)
  def email = column[String]("EMAIL", O.Nullable)
  def phone = column[String]("PHONE", O.Nullable)
  def skype = column[String]("SKYPE", O.Nullable)
  def deptId = column[Int]("DEPT_ID", O.Nullable)
  def dept = foreignKey("EMP_FK",deptId,departments)(_.id)
  def * = (name,last,email.?,phone.?,skype.?,deptId.?, id.?) <> (Employee.tupled, Employee.unapply)
val departments = TableQuery[Departments]

}

这将导致：

List (DepartmentManager(1,"FOO","AAA"),DepartmentManager(3,"XXX","BBB"),DepartmentManager(4,"FOO",""))

---

你在这里遇到的是Slick 2处理连接中空值的笨拙方式。在Slick 3中情况更好，但我们可以在Slick 2中加入工作

您的查询

(departments leftJoin employees ).list

…将是交叉连接，因为您没有连接

使用联接时遇到的错误是因为要选择的列之一为NULL。使用Slick 2，您必须明确选择这些列，并使用来提升它们。？选择一个选项：

val query = 
   departments.leftJoin(employees).on(_.managerId === _.id )
              .map { case (d, e) => (d.id, d.name, e.name.?) }

注意e.name.？-因为该部门可能没有e员工

此查询与测试数据的结果为：

(1, FOO,   Some(AAA))
(2, XXX,   Some(BBB))
(3, dept1, None)

---

…我想这正是您想要的。

您能以表格形式给出您想要的输出示例吗？离题：如果数据库设计尚未解决，我建议使用关系表而不是可为空的外键。这样可以避免一些边缘情况，并简化查询。@Jus12我现在做了，谢谢你的评论。除了SQL之外，表格式将更有帮助。我们需要的是实际的查询和返回的数据。@Jus12它实际上在查询列表departmentmanager下面……谢谢，这正是我要找的。现在清楚多了

val query = 
   departments.leftJoin(employees).on(_.managerId === _.id )
              .map { case (d, e) => (d.id, d.name, e.name.?) }

(1, FOO,   Some(AAA))
(2, XXX,   Some(BBB))
(3, dept1, None)