Slick 动态 groupby

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 slick which groupby 中创建查询并收集给定的列列表。

我在编译时遇到的错误是：

[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）：

你可能已经注意到动态选择列很容易，只要你可以假设一个固定的类型，例如： columnNames = List("col1", "col2") tableQuery.map( r => columnNames.map(name => r.column[String](name)) )

但是如果你try a similar approach 使用groupBy 操作，Slick 会抱怨它"does not know how to map the given types"。

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

1. groupby列类型
2. 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假设一个固定类型，并为所有可能数量的groupBy 列分支代码。

以下是完整的 Scala REPL 会话，可让您了解一下：

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)

当您可以拥有多个 groupBy 列时，这变得难看（即，为 10 多个案例设置单独的 if 分支会变得单调）。希望有人会突然介入并编辑此答案，以了解如何将样板文件隐藏在某些语法糖或抽象层后面。