Linq 根据其他表中的条件选择一个值答案

【问题标题】：Linq select a value based on where condition from other tableLinq 根据其他表中的条件选择一个值
【发布时间】：2016-06-08 20:10:31
【问题描述】：

我有这个问题：

 var result = (from game in db.Games
                    join gameevent in db.Events
                        on game.GameId equals gameevent.GameId into events
                    from _event in events
                    join _targetObjects in db.TargetObjects
                        on _event.TargetObject equals _targetObjects.TargetObjectId into targetss
                    where game.userId == userId
                    select new ProfileViewModel
                    {
                        record = events.Where(s => s.TargetObjectId == _event.TargetObject && _event.EventType == 35).Select(/* Here I want to select a value from targetss, field called TargetName */).ToList()
                    }).First();

如您所见，我想根据其他表中的 where 子句获取值。在选择新部分中可能吗？

我想根据与 events 表中的 targetObjectId 匹配的 targetObjectId 获取 targetObject 的名称，并且事件类型应该是 35。

【问题讨论】：

标签： c# linq linq-to-sql linq-to-entities linq-to-objects

【解决方案1】：

如果查询开始变得过于复杂，则值得将其拆分为多个部分。

在下面的示例中，我使用 LINQ 的扩展方法语法而不是查询关键字，只是因为它更易于使用。

// First we collect the relevant games.
var games =
    db
        .Games
        .Where(game => game.UserId == userId);

// Then we collect the events of the collected games that have the specified event type.
var events = 
    db
        .Events
        .Join(
            games,
            gameEvent => gameEvent.GameId,
            game => game.GameId,
            (gameEvent, game) => gameEvent
        )
        .Where(gameEvent => gameEvent.EventType == 35);

// Then we collect the target objects based on the collected events.
var targetObjects =
    db
        .TargetObjects
        .Join(
            events,
            targetObject => targetObject.TargetObjectId,
            gameEvent => gameEvent.TargetObjectId,
            (targetObject, gameEvent) => targetObject
        );

// Last we select the target name from the collected target objects.
var records =
    targetObjects
        .Select(targetObject => targetObject.TargetName)
        .ToList(); // The query will be executed at this point.

如果这不是您要查找的内容，请说明 ProfileViewModel 应该准确拥有哪些数据以及应该从哪个数据集中选择它，因为我不太熟悉这种语法。

【讨论】：