Scala Play未来的相互依存关系 [英] Scala Play Future Interdependancy

问题描述

我正在为我的scala Play使用play-slick！虚拟休息API.

I am using play-slick for my scala Play! dummy rest API.

因此，我必须从多个表中获取记录.但是，它们是相互依存的，即

So, I have to fetch records from multiple tables. But, they are interdependent, i.e.

Table_1   Table_2
id1       id2
id2

要从Table_2中获取记录，我必须从Table_1中获取记录，然后使用id2从Table_2中获取.

To fetch a record from Table_2 I have to fetch a record from Table_1 and then using id2 fetch from Table_2.

我的控制器:

def getEntity(id : Long) = Action.async {
  table1DAO.findById(id) map { t1 =>
    t1 map { t1Entity =>
      table2DAO.findById(t1Entity.id2) map { t2 =>
        t2 map { t2Entity =>
          Ok(Json.toJson(CombiningClass(t1Entity, t2Entity)))
        } getOrElse { Ok(Json.toJson(t1Entity)) }
      }
    } getOrElse { NoContent }
  }
}

编译后，我得到:

[error] DummyController.scala:17: overloaded method value async with alternatives:
[error]   [A](bodyParser: play.api.mvc.BodyParser[A])(block: play.api.mvc.Request[A] => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[A] <and>
[error]   (block: play.api.mvc.Request[play.api.mvc.AnyContent] => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[play.api.mvc.AnyContent] <and>
[error]   (block: => scala.concurrent.Future[play.api.mvc.Result])play.api.mvc.Action[play.api.mvc.AnyContent]
[error]  cannot be applied to (scala.concurrent.Future[Object])
[error]   def getEntity(id : Long) = Action.async {
[error]                                ^
[error] one error found

这是我的DAO方法:

def findById(id : Long): Future[Option[A]] = {
  db.run(tableQ.filter(_.id === id).result.headOption)
}

PS:我对功能范式和Scala还是很陌生，所以，如果可以的话，请原谅我的无知.

PS: I'm very new to functional paradigm and scala, so if you can, pardon my ignorance.

推荐答案

要简单地解决您的问题，

To simply solve your problem:

def getEntity(id : Long) = Action.async {
  findById(id) flatMap {
    case Some(t1Entity) =>
      findById(t1Entity.id2) map { t2Opt =>
        t2Opt map { t2Entity =>
          Ok(Json.toJson(t1Entity, t2Entity))
        } getOrElse { Ok(Json.toJson(t1Entity)) }
      }
    case None => Future.successful(NoContent)
  }
}

这里的问题是您不能在Scala中一起flatMap Option和Future. 此处是关于该主题的精彩而简单的文章(带有自定义FutureO monad实现作为解决方案).长话短说，我会使用 cats 库(甚至是scalaz库)和OptionT功能.我稍微简化了您的代码.

The problem here is that you can't flatMap Option and Future together in scala. Here is a fantastic and simple article concerning this topic (with the custom FutureO monad implementation as a solution). Long story short, I would use the cats library (or even scalaz library) and OptionT feature. I slightly simplified your code.

def getEntity(id : Long) = Action.async {
  (for {
    t1 <- daoFindById(id)
    t2 <- daoFindById(t1.id2)
  } yield (t1, t2)).map{
    result => Ok(Json.toJson(result))
  }.getOrElse(NoContent)
}

case class T(id2: Long)
def daoFindById(id : Long): OptionT[Future, T] = {
  OptionT[Future, T](db.run(tableQ.filter(_.id === id).result.headOption))
}

您现在可以轻松地在该OptionT单子上使用flatMap，并且不管您是在处理Option还是Future(因为对scala的理解只是语法糖).

You can now easily flatMap over this OptionT monad, and don't care if you are dealing with Option or Future (for comprehension in scala is only a syntactic sugar).

这篇关于Scala Play未来的相互依存关系的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！