Question

我有以下查询，它返回一个客人列表以及一系列RSVP。但是，由于用户每个事件可以有多个RSVP，我只想获得最新的。 RSVP表具有提交的日期列。

from guest in Guests
                join rsvp in RSVPs.Where(o => o.EventID == "1234")
                on guest.UserName equals rsvp.UserName into sr
                from x in sr.DefaultIfEmpty()
                select new
                {
                     guest,
                    x
                }

Answer 1

按日期排序并取1：

(from guest in Guests
            join rsvp in RSVPs.Where(o => o.EventID == "1234")
            on guest.UserName equals rsvp.UserName into sr
            from x in sr.DefaultIfEmpty()
            orderby x.RSVPDate desc
            select new
            {
                 guest,
                x
            })
.Take(1);

Answer 2

假 -

group x by username into g
select new {
  UserName = g.key // username
  FirstRsvp = g.OrderByDescending(item => item.RsvpDate).FirstOrDefault()
}

希望这是对象的LINQ，因为这种事情可能无法在SQL提供程序中定义。如果是SQL并且没有实现，.AsEnumerable（）会将查询拉入内存，然后从那里开始是L2O。

Answer 3

from guest in Guests
                join rsvp in RSVPs.Where(o => o.EventID == "1234")
                on guest.UserName equals rsvp.UserName into sr
                from x in sr.DefaultIfEmpty()
                select new
                {
                     guest,
                    guest.RSVPs.Where(o => o.EventID =="1234")
                               .OrderByDesc(o=>o.SubmittedDate)
                               .FirstOrDefault()
                }

实际上，你也可以尝试这个，想法是你加载正确的RSVP并加载附加的guest对象，因为有一个外键关系。

context.RSVPs.Include("Guest").Where(o => o.EventID =="1234").OrderByDesc(o=>o.SubmittedDate).FirstOrDefault()

否则这可能是一个选择：

(from rsvp in context.RSVP
where rsvp.EventId == "1234"
join guest in context.Guests on rsvp.UserName equals guest.UserName 
select new {rsvp, guest}
orderby desc rsvp.SubmittedDate).FirstOrDefault()

LINQ查询 - 按键获取最新记录

3 个答案: