Linq2Db / Access - error with anonymous group by multiple fields - vb.net

Question

I have 2 tables: pob (with results of some activities) and names (with user data). I try to select top 5 users from table pob based on their last activity date. So I inner join names and pob, then select top 5 users based on calculated max(date).
This query is working:
SELECT TOP 5
[u].[id],
[u].[name],
max([p].[date]) As LastDateOfUse,
FROM
[pob] [p]
INNER JOIN
[users] [u]
ON
[p].[id_name] = [u].[id]
WHERE
[p].[date] >= #2017-01-01#
GROUP BY
[u].[id],
[u].[name]
ORDER BY
max([p].[date]) DESC
Now I need to transform it into Linq query. This my attempt but it's not working.
"Key" is not a member of type "System.Collections.Generic.IEnumerable'1[VB$AnonymousType_2'2[pob,users]]".
Using db = New DbContext() With {.InlineParameters = True}
Dim query1 = From p In db.pob
Join u In db.users On p.id_name Equals u.id
Where p.date >= New Date(2017, 1, 1)
Group New With {p, u} By pu = New With {Key u.id, Key u.name} Into pg = Group
Select New RecentUser With
{
.id = pu.id,
.name = pu.name,
.LastDateOfUse = pg.Max(Function(f) f.p.date)
}
query1 = query1.OrderByDescending(Function(f) f.LastDateOfUse).Take(5)
Return query1.ToList
End Using
If I remove .LastDateOfUse = pg.Max(Function(f) f.p.Date) like below it works. By 'works' I mean there is no exception but of course result of query is wrong however grouping is done properly.
Using db = New DbContext() With {.InlineParameters = True}
Dim query1 = From p In db.pob
Join u In db.users On p.id_name Equals u.id
Where p.date >= New Date(2017, 1, 1)
Group New With {p, u} By pu = New With {Key u.id, Key u.name} Into pg = Group
Select New RecentUser With
{
.id = pu.id,
.name = pu.name
}
Return query1.ToList
End Using
Edit
I also tried going through navigation properties like below, but again I receive the same error.
Using db = New DbContext() With {.InlineParameters = True}
Dim query1 = From p In db.pob
Where p.date >= New Date(2017, 1, 1)
Group p By pu = New With {Key u.User.id, Key u.User.name} Into pg = Group
Select New RecentUser With
{
.id = pu.id,
.name = pu.name
.LastDateOfUse = pg.Max(Function(f) f.date)
}
query1 = query1.OrderByDescending(Function(f) f.LastDateOfUse).Take(5)
Return query1.ToList
End Using
And again if I remove .LastDateOfUse = pg.Max(Function(f) f.p.Date) like below it starts to work (proper grouping, wrong overall result).
Using db = New DbContext() With {.InlineParameters = True}
Dim query1 = From p In db.pob
Where p.date >= New Date(2017, 1, 1)
Group p By pu = New With {Key u.User.id, Key u.User.name} Into pg = Group
Select New RecentUser With
{
.id = pu.id,
.name = pu.name
}
Return query1.ToList
End Using
How can I transform above Sql query to Linq? (preferable answer in VB.Net but C# is ok too)

Answer 1

Solution
There is no solution yet. It looks like VB has bad Linq queries resolver - it creates unexpected method chain that can not be converted to SQL.
So instead
Group By ... Into pg = Group
we need
Group By ... Into LastDateOfUse = p.Max(Function(f) f.date).
See below full query.
Using db = New DbContext() With {.InlineParameters = True}
Dim query1 = From p In db.pob
Where p.date >= New Date(2017, 1, 1)
Group p By pu = New With {Key u.User.id, Key u.User.name} Into LastDateOfUse = p.Max(Function(f) f.date)
Select New RecentUser With
{
.id = pu.id,
.name = pu.name
.LastDateOfUse = LastDateOfUse
}
Return query1.ToList
End Using
Another problem with lambda syntax
Using lambda syntax we receive another exception.
Dim query = db.pob.
Where(Function(f) f.date >= New Date(2017, 1, 1).
GroupBy(Function(f) New With
{
Key .userid= f.user.id,
Key .username = f.user.name
}).Select(Function(f) New RecentUser With
{
.id = f.Key.userid,
.name = f.Key.username,
.LastDateOfUse = f.Max(Function(g) g.date)
}).ToList
Exception
VB.NET compiler adds unnecessary Convert to IEnumerable when generating Expression Tree.
An unhandled exception of type LinqToDB.Linq.LinqException occurred in linq2db.dll
Convert(f).Select(g => g.Date).Max() cannot be converted to SQL
GitHub
I posted an issue here.
Svyatoslav Danyliv based on my issue opened his own here.