在更新一个光滑表的前n行(Update top n rows of a table in slick

我想用的时候更新表不是整个一个顶n行slick 3.0

这是更新所有版本：

private[this] val active = this.filter(a => a.status =!= AccountStatus.DISABLED)
db.run(
    active.filter(a => a.usedBy.isEmpty || a.usedBy === Host.name)
    .map(account => account.usedBy)
    .update("host-a")
)

我试图用这个版本，但它没有工作和抛出的异常

private[this] val active = this.filter(a => a.status =!= AccountStatus.DISABLED)
db.run(
    active.filter(a => a.usedBy.isEmpty || a.usedBy === Host.name)
    .take(10)
    .map(account => account.usedBy)
    .update(Option(Host.name))
)

例外

Caused by: slick.SlickException: A query for an UPDATE statement must resolve to a comprehension with a single table -- Unsupported shape: Comprehension s2, Some(Apply Function and), None, ConstArray(), None, None, Some(LiteralNode 100 (volatileHint=false)), None
at slick.driver.JdbcStatementBuilderComponent$QueryBuilder.buildUpdate(JdbcStatementBuilderComponent.scala:447)
at slick.driver.JdbcProfile$$anonfun$updateCompiler$1.apply(JdbcProfile.scala:30)
at slick.driver.JdbcProfile$$anonfun$updateCompiler$1.apply(JdbcProfile.scala:30)
at slick.jdbc.JdbcMappingCompilerComponent$JdbcCodeGen.compileServerSideAndMapping(JdbcMappingCompilerComponent.scala:59)

嗯......这个问题的答案在于，事实上，你正在尝试做一些东西，油滑的是不应该做的事。

一个非常简单的指引 - 有疑问时认为，在SQL然后变换到油滑

试想你将如何在SQL实现这一点，

如果我改变你的“查询”，

// lets say Host.name = "Awesome-Host"

active.filter(a => a.usedBy.isEmpty || a.usedBy === Host.name)
  .take(10)
  .map(account => account.usedBy)
  .update(Option(Host.name))

到SQL，它会是这样的，

UPDATE
    active
SET
    used_by = 'Awesome-Host'
WHERE
    used_by IS NULL
    OR used_by = 'Awesome-Host'
LIMIT 10

这是一种荒谬从SQL角度来看...

现在...让我们来谈谈如何将你实际上SQL做到这一点，

UPDATE
    (
        SELECT 
            *
        FROM
            active
        WHERE
            used_by IS NULL
            OR used_by = 'Awesome-Host'
        LIMIT 10
    ) active_selection
SET
    active_selection.used_by = 'Awesome-Host'

...这可以翻译成油滑使用子查询

val activeSelection = active
  .filter(a => a.usedBy.isEmpty || a.usedBy === Host.name)
  .take(10)

val updateSelection = activeSelection
  .map(a => a.usedBy)
  .update(Option(Host.name))