Question

我正在搜索我在stackoverflow（Get count of rows in table A that have a reference to table B）

上找到的特定SQL

但是我似乎无法在Slick中实现这个SQL解决方案。

SELECT device.*, c.cnt
 FROM device 
 LEFT JOIN (SELECT device_id, COUNT(*) AS cnt 
             FROM unit 
             GROUP BY device_id) c 
 ON device.id = c.device_id

我当前的方法失败了

slick.SlickException: Unexpected node Select _2 -- SQL prefix: select x2.x3, x2.x4, x2.x5, x2.x6,

尝试运行/编译查询时

val devicesWithUnitCountResult = dbConfig.db.run(TableQueries.devicesWithUnitCount.result)

当前代码：

  def unitCountPerDevice = for {
    (id, length) <- TableQuery[TDUnit].groupBy(k => k.deviceID).map(k => (k._1, k._2.map(_.id).length))
  } yield (id, length)

  def devicesWithUnitCount = for {
    (device, unitcount) <- TableQuery[TDDevice].joinLeft(unitCountPerDevice).on(_.id === _._1).map(k => {
      val u = k._2.getOrElse((0, 0))._2
      (k._1, u)
    })
  } yield (device, unitcount)

有人可以帮助我找出我在这里失败的地方吗？

Answer 1

以这种方式试试

def unitCountPerDevice = TableQuery[TDUnit].groupBy(k => k.deviceID).map(k => (k._1, k._2.map(_.id).length))

def devicesWithUnitCount = TableQuery[TDDevice].joinLeft(unitCountPerDevice).on(_.id === _._1).map(d =>(d._1,d._2.map(_._2)))

在Scala Slick中实现复杂的左连接

1 个答案: