Question

我有这样的代码：

def query(buckets: List[String]): Future[Seq[(List[Option[String]], Option[Double])]] = {
    database.run {
        groupBy(row => buckets.map(bucket => customBucketer(row.metadata, bucket)))
            .map { grouping =>
                val bucket = grouping._1
                val group = grouping._2
                (bucket, group.map(_.value).avg)
            }
            .result
    }
}

private def customBucketer(metadata: Rep[Option[String]], bucket: String): Rep[Option[String]] = {
    ...
}

我希望能够在光滑的分组中创建查询，并在给定的列列表上进行收集。

我正在编译的错误是：

[error] Slick does not know how to map the given types.
[error] Possible causes: T in Table[T] does not match your * projection,
[error]  you use an unsupported type in a Query (e.g. scala List),
[error]  or you forgot to import a driver api into scope.
[error]   Required level: slick.lifted.FlatShapeLevel
[error]      Source type: List[slick.lifted.Rep[Option[String]]]
[error]    Unpacked type: T
[error]      Packed type: G
[error]                         groupBy(row => buckets.map(bucket => customBucketer(row.metadata, bucket)))
[error]                                ^

Answer 1

这是Slick 3.2.3的解决方法（以及我的方法的一些背景知识）：

您可能已经注意到动态选择列很容易，只要您可以采用固定类型，例如： columnNames = List("col1", "col2") tableQuery.map( r => columnNames.map(name => r.column[String](name)) )

但如果try a similar approach进行groupBy操作，Slick会抱怨"does not know how to map the given types"。

所以，虽然这不是一个优雅的解决方案，但你至少可以通过静态定义两者来满足Slick的类型安全性：

groupby列类型
groupBy列数量的上限/下限

实现这两个约束的一种简单方法是再次采用固定类型并为所有可能数量的groupBy列分支代码。

这是一个完整的Scala REPL会话，可以给你一个想法：

import java.io.File

import akka.actor.ActorSystem
import com.typesafe.config.ConfigFactory
import slick.jdbc.H2Profile.api._

import scala.concurrent.{Await, Future}
import scala.concurrent.duration._


val confPath = getClass.getResource("/application.conf")
val config = ConfigFactory.parseFile(new File(confPath.getPath)).resolve()
val db = Database.forConfig("slick.db", config)

implicit val system = ActorSystem("testSystem")
implicit val executionContext = system.dispatcher

case class AnyData(a: String, b: String)
case class GroupByFields(a: Option[String], b: Option[String])

class AnyTable(tag: Tag) extends Table[AnyData](tag, "macro"){
  def a = column[String]("a")
  def b = column[String]("b")
  def * = (a, b) <> ((AnyData.apply _).tupled, AnyData.unapply)
}

val table = TableQuery[AnyTable]

def groupByDynamically(groupBys: Seq[String]): DBIO[Seq[GroupByFields]] = {
  // ensures columns are returned in the right order
  def selectGroups(g: Map[String, Rep[Option[String]]]) = {
    (g.getOrElse("a", Rep.None[String]), g.getOrElse("b", Rep.None[String])).mapTo[GroupByFields]
  }

  val grouped = if (groupBys.lengthCompare(2) == 0) {
    table
      .groupBy( cols => (cols.column[String](groupBys(0)), cols.column[String](groupBys(1))) )
      .map{ case (groups, _) => selectGroups(Map(groupBys(0) -> Rep.Some(groups._1), groupBys(1) -> Rep.Some(groups._2))) }
  }
  else {
    // there should always be at least one group by specified
    table
      .groupBy(cols => cols.column[String](groupBys.head))
      .map{ case (groups, _) => selectGroups(Map(groupBys.head -> Rep.Some(groups))) }
  }

  grouped.result
}

val actions = for {
  _ <- table.schema.create
  _ <- table.map(a => (a.column[String]("a"), a.column[String]("b"))) += ("a1", "b1")
  _ <- table.map(a => (a.column[String]("a"), a.column[String]("b"))) += ("a2", "b2")
  _ <- table.map(a => (a.column[String]("a"), a.column[String]("b"))) += ("a2", "b3")
  queryResult <- groupByDynamically(Seq("b", "a"))
} yield queryResult

val result: Future[Seq[GroupByFields]] = db.run(actions.transactionally)
result.foreach(println)

Await.ready(result, Duration.Inf)

如果这种情况变得丑陋，那么当你可以拥有一些groupBy列时（即在10个以上的情况下有一个单独的if分支会变得单调）。希望有人会为了如何隐藏一些语法糖或抽象层背后的样板而匆匆进入并编辑这个答案。

光滑的动态组合

1 个答案: