Query: Property binding on the result entity from grouping.First

[smitpatel]

Query like

dbset.GroupBy(p=> new { MyField = p.MyField, MyOtherField = p.MyOtherField}).Select(p=> p.First()).OrderBy(p=> p.MyField)

MyField is mapped property but in translation we see a shaped query on which we have to bind rather than entity reference so we fail to bind and throw error that OrderBy couldn't be translated. (Same goes for Select or any other operation which tries to bind property)

[OpenSpacesAndPlaces]

Noting for my use case - pre-OrderBy worked ok (I guess that's a workaround of sorts).

Count or Select Shaping specifically were the issues:
GroupBy(p=> new { MyField = p.MyField, MyOtherField = p.MyOtherField}).Select(p=> p.First()).Count()
GroupBy(p=> new { MyField = p.MyField, MyOtherField = p.MyOtherField}).Select(p=> p.First()).Select(p=>p.MyField)

[smitpatel]

I ran the query with OrderBy in release/6.0 branch and it failed for me with client eval error. Select also caused client eval error rather than a random exception. Please provide a runnable repro if you are seeing otherwise.

[OpenSpacesAndPlaces]

I see why - it was getting applied before the GroupBy:
dbset.OrderBy(p=> p.MyField).GroupBy(p=> new { MyField = p.MyField, MyOtherField = p.MyOtherField}).Select(p=> p.First())

[LeaFrock]

Another use case of mine,

Id	Name	Age	Country
1	Alice	12	Russia
2	Bob	11	China
3	Millie	18	America
4	Peter	29	China
5	John	20	America
6	Mike	17	Russia

What I want to do,

1. Group by Country
2. Take people with the largest Age(but < 25) of each group
3. Each taken person only select Id,Name
4. Return only two oldest people in a desc order of Age

So, My codes are like below,

            var result = await context.People
                .Where(p => p.Age < 25)
                .GroupBy(r => r.Country)
                .Select(g => g
                    .OrderByDescending(p => p.Age)
                    .First())
                .OrderByDescending(p => p.Age)
                .Take(2)
                .Select(p => new Person()
                {
                    Id= p.Id,
                    Name=p.Name
                })
                .ToListAsync();

                // ....

But now EF Core throws InvalidOperationException: ... could not be translated. ....

[OpenSpacesAndPlaces]

@LeaFrock Generically - do as much as you can before the GroupBy and then anything that can't, client-side the effort after.
They are targeting numerous fixes for GroupBy in 7.0, but for now, it's workarounds.

Right off the bat I can see this after the GroupBy - that alone will throw the error you're getting:
.OrderByDescending(p => p.Age)