如何分组/聚合但返回汇总字段以外的字段？

Question

我有两个表，任务和TaskMilestones。我想要查询返回每个任务的最近过去的里程碑和最近的未来TaskMilestone。我正在使用C＃/ LINQ到SQL。我如何去做这件事？

任务栏：ID，TASKNAME TaskMilestones列：ID，任务id，MilestoneName，MilestoneDate

我想与含有行的返回表：TASKNAME，MilestoneDate，MilestoneName

我目前的解决方案导致的LINQ查询每个任务的数据库一次，这是无法接受的缓慢。

[编辑处理意见]当前的实现很简单，不是一个单一的语句，而只是用适当的where子句查询任务列表，然后为每个任务id查询两次：

var x = from p in this.Database.Task 
     join pm in this.Database.TaskMilestones on p.Id equals pm.TaskId 
     select new 
     { 
      TaskId = p.Id, 
      TaskName = p.Name, 
      MilestoneName = m.Name, 
      MilestoneDate = pm.MilestoneDate, 
     }; 

foreach (var record in records) 
{ 
    var y = x.Where(p => p.TaskId == record.Id && p.MilestoneDate <= dt); 
    var z = x.Where(p => p.TaskId == record.Id && p.MilestoneDate > dt); 

    ... 

Answer 1

DateTime dt = DateTime.Today; 

var records = 
    from p in db.Tasks 
    let pastMilestone = p.TaskMilestones 
    .Where(pm => pm.MilestoneDate <= dt) 
    .OrderByDescending(pm => pm.MilestoneDate) 
    .FirstOrDefault() 
    let nextMilestone = p.TaskMilestones 
    .Where(pm => pm.MilestoneDate > dt) 
    .OrderBy(pm => pm.MilestoneDate) 
    .FirstOrDefault() 
    select new 
    { 
    Task = p, 
    PastMilestone = pastMilestone, 
    NextMilestone = nextMilestone 
    } 

---

另一种选择是加载所有的里程碑为电子ach项目，然后使用LinqToObjects对其进行过滤：

DataLoadOptions dlo = new DataLoadOptions(); 
dlo.LoadWith<Task>(p => p.TaskMilestones); 
db.LoadOptions = dlo; 

var records = db.Tasks; 

foreach(Task record in records) 
{ 
    TaskMilestone pastMilestone = record.TaskMilestones 
     .Where(pm => pm.MilestoneDate <= dt) 
     .OrderByDescending(pm => pm.MilestoneDate) 
     .FirstOrDefault() 
    TaskMilestone nextMilestone = record.TaskMilestones 
     .Where(pm => pm.MilestoneDate > dt) 
     .OrderBy(pm => pm.MilestoneDate) 
     .FirstOrDefault() 
} 

Answer 2

关闭顶部我的头：

var q = from t in Context.Tasks 
     let mostRecentDate = t.Milestones.Where(m => m.MilestoneDate < DateTime.Now) 
             .Max(m => m.MilestoneDate) 
     select new 
     { 
      TaskId = t.Id, 
      TaskName = t.TaskName, 
      RecentPastMilestone = t.Milestones.Where(m => m.MilestoneDate == mostRecentDate) 
               .FirstOrDefault() 
     };