EFCore - Select 分组后

Question

List<int> data = dbContext.table
.GroupBy(x => x.Date)
.OrderByDescending(x => x.Key)
.Take(1)
.SelectMany(x => x.Select(c => c.Id))
.ToList();

With this query, i want to get all ids with the latest date. But I get an error for this query using EFCore.(But this works just fine using EF not EFCore). Error says;

"The LINQ expression... could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'."

I am curious why it works in Ef but not EFCore?

Answer 1

In SQL a GROUP BY clause collapses rows and returns a single row per group. That row can only contain the group columns and aggregate rows. .SelectMany(x => x.Select(c => c.Id)) doesn't make sense in a GROUP BY query.

I suspect the query is trying to retrieve the IDs for the latest date. In SQL that requires a subquery to find that latest date:

SELECT ID
FROM TheTable
Where Date=(SELECT MAX(Date) 
            FROM TheTable)

The equivalent in EF would be:

var ids=dbContext.TheTable
                 .Where(t=>t.Date==dbContext.TheTable.Max(r=>r.Date))
                 .Select(t=>t.Id)
                 .ToList();