How to Filter Records After GroupBy with LINQ + EF Core 6 - linq

Question

Given this simple Contract entity and assuming that a customer can have many Contracts with different start dates and statuses.
public class Contract {
[Key]
int ContractId { get; set; }
int CustomerId { get; set; }
string Status { get; set; }
DateTime ContractStartDate { get; set; }
}
I'm trying to write a query to find that latest contract for each customer and then filter out certain statuses. I need to filter the status after finding the latest contract, to ensure I'm getting their current status.
ContractID
CustomerID
ContractStartDate
Status
1
1
2022-01-01
Active
2
1
2022-31-05
Inactive
3
2
2022-01-03
Active
4
2
2022-31-07
Inactive
From the above data set, I would expect to get contracts 2 and 4 in the results. This is why I can't filter on status before grouping, because then the latest inactive row would be eliminated before I group them by customer to find the latest row.
I've tried something like, this:
var latestContracts = Query<Contract>()
.GroupBy(grp => grp.CustomerId)
.Select(s => s.OrderByDescending(s => s.ContractStartDate).First())
.Where(w => w.Status == "Active");
but once I execute the query by calling ToListAsync(), etc. I get an error like this:
'The LINQ expression 'DbSet<Customer>()
.GroupBy(c => c.CustomerId)
.Select(g => g
.AsQueryable()
.OrderByDescending(e => e.
I CC ContractStartDate)
.First())
.Where(e0 => e0.AccountContractTermStatus == "Active")' 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'. See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.'
If I remove the Where call, the query works fine and gives the intended newest results. I know that the support for translating LINQ to SQL is still not fully complete, even in EF Core 6, but this seems like something that should be possible. Am I missing something simple or is this just not yet doable via LINQ?
Happy to provide more context, as I've greatly simplified a production query into something I could share publicly.

Answer 1

Try the following emulation of query which EF Core cannot translate. It is almost direct translation what EF Core should do in your case.
var dataQuery = Query<Contract>();
var latestContracts =
from d in dataQuery.Select(d => new { d.CustomerId }).Distinct()
from c in dataQuery
.Where(c => d.CustomerId == c.CustomerId)
.OrderByDescending(c => c.ContractStartDate)
.Take(1)
where c.Status == "Active"
select c;
Or using Method Chain syntax:
var dataQuery = Query<Contract>();
var latestContracts = dataQuery
.Select(d => new { d.CustomerId })
.Distinct()
.SelectMany(d => dataQuery
.Where(c => d.CustomerId == c.CustomerId)
.OrderByDescending(c => c.ContractStartDate)
.Take(1))
.Where(c => c.Status == "Active");