如何将未来[向量[UserLocation]]转换为未来[向量[User]]

Question

我有一个方法，我想返回Future[Vector[user]]。

方法userLocationService.getUserLocationsInList将返回Future[Vector[UserLocation]]。

其中用户位置看起来像：

case class UserLocation(id: Int, locationId: Int, userId: Int) 


def getUsersInLocation(locationIdList: Set[Int]): Future[Vector[User]] = { 

    userLocationService.getUserLocationsInList(locationIdList).map{ 
     userLocations => 
      // ???????????? 
    } 

} 

我有一个基于用户ID喜欢返回单个用户的方法：

userService.getById(userId: Int): Future[User] 

如何建立一个未来[矢量[用户]鉴于上述？

Answer 1

如果map的Future[Vector[UserLocation]]，你可以很容易产生内Vector[Future[User]]，从包含的Vector[UserLocation]：

userLocations.map(location => userService.getById(location.userId)) 

您可以使用Future.sequence反转Vector[Future[User]]到Future[Vector[User]]：

Future.sequence(userLocations.map(location => userService.getById(location.userId))) 

或者使用Future.traverse：

Future.traverse(userLocations) { location => userService.getById(location.userId) } 

这会给你一个Future[Future[Vector[User]]]，可以通过将map更改为flatMap来修复。全部放在一起：

def getUsersInLocation(locationIdList: Set[Int]): Future[Vector[User]] = { 
    userLocationService.getUserLocationsInList(locationIdList).flatMap { locations => 
    Future.traverse(locations) { location => 
     userService.getById(location.userId) 
    } 
    } 
} 

或者用换理解：

def getUsersInLocation(locationIdList: Set[Int]): Future[Vector[User]] = { 
    for { 
    locations <- userLocationService.getUserLocationsInList(locationIdList) 
    users <- Future.traverse(locations) { location => 
     userService.getById(location.userId) 
    } 
    } yield users 
}