如何分组/聚合但返回聚合字段以外的字段？

Question

我有两个表，Tasks 和 TaskMilestones。我想要一个查询来为每个任务返回最近的过去里程碑和最近的未来 TaskMilestone。我正在使用 C#/LINQ-to-SQL。我该怎么做？

任务列：Id、TaskName TaskMilestones 列：Id、TaskId、MilestoneName、MilestoneDate

我想要一个包含以下行的返回表：TaskName、MilestoneDate、MilestoneName

我当前的解决方案导致 Linq 为每个任务查询一次数据库，这非常慢。

[编辑解决 cmets] 当前的实现很简单，不是一条语句，它只是查询任务列表，然后使用适当的 where 子句查询每个 TaskId 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
  }

---

另一种选择是加载每个项目的所有里程碑，然后使用 LinqToObjects 过滤它们：

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

在我的头顶：

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()
        };