按 linq 分组到实体查询，通过加入表获得一条具有最新时间戳的记录

Question

There are two tables and using linq query to get records. From second table, there can be multiple rows corresponding to first table with date timestamp... based on below query, I am getting all records, but is there a way we can get the row from second table which has latest timestamp?

Table Parent

ID            Name    
1              M            
2              N             
3              O
4              P
5              Q

Table Child
Id            fkID        DateTime
1              2              01/12/2021 09:12:20
2              2              01/12/2021 09:13:20
3              2              01/12/2021 09:14:20
4              2              01/12/2021 09:15:20
5              2              01/12/2021 **09:16:20**

Linq query:

from p in Parent
join c in Child on p.id equals c.fkId into cJoin
from cJoin in cJoin.DefaultIfEmpty()
select new TempResponse
{
Id = p.Id,
Name = p.Name,
Date = c.Date
}

I am getting 10 records using above query but just need 5 records i.e. from child table instead of all 5 records, we need a record that has latest time stamp
 
**expected output**
1              M
2              N             01/12/2021 09:16:20 
this record is 5'th record from child table because this one has latest date time stamp 
( latest record )
3              O
4              P
5              Q

Is there any way we can use group by and get the record that has latest time stamp from second table?

Answer 1

Assuming you have defined navigation properties for the FK, I would use a query like;

dbContext.Child.Where(c => c.Date == c.Parent.Child.Max(c2 => c2.Date))

Answer 2

I believe you can use:

var ans = from p in Parent
          join cmax in (
            from c in Child
            group c by c.fkId into cg
            select cg.OrderByDescending(c => c.Date).FirstOrDefault())
          on p.Id equals cmax.fkId into cJoin
          from c in cJoin.DefaultIfEmpty()
          select new TempResponse {
              Id = p.Id,
              Name = p.Name,
              Date = c != null ? c.Date : null
          };

Note that the order of results seems to vary on SQL Server unless you add another orderby clause before the select to force an order.