The right way to apply GroupBy extension method with aggregate function - entity-framework

Question

I have this simple model.
public class Room
{
public Guid Id { get; set; }
public Guid? postSubjectId { get; set; }
[ForeignKey("postSubjectId")]
public PostSubject postSubject { get; set; }
public string MemberId { get; set; }
[ForeignKey("MemberId")]
public AppUser Member { get; set; }
}
Basically I need to get Grouped postSubjectId along with MemberId.Count() , I know it's easy .. but it never comes with the expected result.
I made this simple GroupBy query
var mmbrs = _context.Rooms
.Select(g => new { id = g.postSubjectId, mmbrscount = g.MemberId })
.AsEnumerable()
.GroupBy(g => new { id = g.id , mmbrscount = g.mmbrscount.Count() }).ToList();
but it gives me unexpected result
However I did the same using ordinary sql query
select [postSubjectId] as postId, count([MemberId]) as mmbrsCount from [dbo].[Rooms] group by [postSubjectId]
and It gives me result as expected
I need to apply that expected result using LINQ GruoupBy extention method

Answer 1

The grouping key new { id = g.postSubjectId, mmbrscount = g.MemberId }) is like typing group by [postSubjectId], count([MemberId]) in SQL.
The correct statement is:
_context.Rooms
.GroupBy(r => r.postSubjectId)
.Select(g => new
{
id = g.Key,
mmbrscount = g.Count()
})

Answer 2

So every Room has exactly one property PostSubjectId, and one string property MemberId.
I need to get Grouped postSubjectId along with MemberId.Count()
Apparently you want to make groups of Rooms that have the same value for property PostSubjectId AND have the same value for MemberId.Count().
var result = dbContext.Rooms.GroupBy(room => new
{
PostSubjectId = room.PostSucjectId,
MemberIdLength = room.MemberId.Count(),
});
The result is a sequence of groups of Rooms. Every group has a key, which is a combination of [PostSubjectId, MemberIdLength]. The group is a sequence of Rooms. All rooms in one group have the same combination of [PostSubjectId, MemberIdLength].
If you don't want a sequence of groups of Rooms, you can use the overload of GroupBy that has a parameter resultSelector
var result = dbContext.Rooms.GroupBy(
// parameter keySelector
room => new
{ PostSubjectId = room.PostSucjectId,
MemberIdLength = room.MemberId.Count(),
},
// parameter resultSelector:
// from every combination of [PostSubjectId, MemberIdLength] (= the key) and
// all rooms that have this combination, make one new object:
(key, roomsWithThisKey) => new
{
// select the properties that you actually plan to use, for example
PostSubjectId = key.PostSubjectId,
MemberIdLength = key.MemberIdLength,
RoomInformations = roomsWithThisKey.Select(roomWithThisKey => new
{
Id = roomWithThisKey.Id,
Member = roomWithThisKey.Member,
...
})
.ToList(),
});