Question

代码如下所示：

case class Supplier(snum: String, sname: String, status: Int, city: String)

class Suppliers(tag: Tag) extends Table[Supplier](tag, "suppliers") {
  def snum  = column[String]("snum")
  def sname = column[String]("sname")
  def status   = column[Int]("status")
  def city     = column[String]("city")
  def * = (snum, sname, status, city) <> (Supplier.tupled, Supplier.unapply _)
}

val suppliers = TableQuery[Suppliers]

val gr=suppliers.groupBy(_.city).map{ case (k,v) => (k, v) }.buildColl[Set]

当我编译它时，它会抱怨：

Error:(69, 43) No matching Shape found.
Slick does not know how to map the given types.
Possible causes: T in Table[T] does not match your * projection. Or you use an unsupported type in a Query (e.g. scala List).
  Required level: scala.slick.lifted.FlatShapeLevel
     Source type: (scala.slick.lifted.Column[String], scala.slick.lifted.Query[A$A113.this.Suppliers,A$A113.this.Supplier,[+A]Seq[A]])
   Unpacked type: T
     Packed type: G
lazy val gr=suppliers.groupBy(_.city).map{ case (k,v) => (k, v) }.buildColl[Set]
                      ^
Error:(69, 43) not enough arguments for method map: (implicit shape: scala.slick.lifted.Shape[_ <: scala.slick.lifted.FlatShapeLevel, (scala.slick.lifted.Column[String], scala.slick.lifted.Query[A$A113.this.Suppliers,A$A113.this.Suppliers#TableElementType,Seq]), T, G])scala.slick.lifted.Query[G,T,Seq].
Unspecified value parameter shape.
lazy val gr=suppliers.groupBy(_.city).map{ case (k,v) => (k, v) }.buildColl[Set]
                      ^

但如果我将case(k,v)=>(k,v)更改为case(k,v)=>(k,v.length)，则会再次有效。

有没有人有这方面的想法？

Answer 1

原因是：Scala的groupBy返回Map[..., Seq[...]]，换言之，包含其他集合的集合。嵌套的集合！但SQL不支持嵌套集合，它总是返回平面表。支持嵌套集合需要比Slick目前更复杂的从Scala到SQL的转换。因此，Slick禁止这种情况，并要求你让它平坦。例如，case(k,v)=>(k,v.length)会将其转换为Map[..., Int]。 Slick通过说Required level: scala.slick.lifted.FlatShapeLevel来告诉你。

解决方法是在客户端上进行分组，例如suppliers.run.groupBy(_.city)或在Slick 2.2及更高版本db.run(suppliers).groupBy(_.city).如果是加入，运行两个查询并在本地加入它们可能会更有效，而不是转移笛卡尔积并在之后进行分组。

groupBy方法在Slick中抛出错误

1 个答案: