Take each first element of the group by - entity-framework

Question

How can I take each first element in the group with EF 5 ?
var result = await context.SomeDbSet
.Where(...)
.GroupBy(x => new { x.SomeField, ... })
.Select(x => x.First())
.ToListAsync();
I am getting not supported exception.
How to correctly rewrite query? Thanks.

Answer 1

You cannot do that with grouping. SQL has a limitation - with GROUP BY you can select only grouping keys and aggregation result. This limitation for sure extended to LINQ to Entities - after GroupBy you can select only grouping keys and aggregation result.
Such result can be achieved by SQL and Window functions:
SELECT
r.*,
FROM
(
SELECT
s.*,
ROW_NUMBER() OVER(PARTITION BY s.SomeField1, s.SomeField2 ORDER BY s.SomeDate) AS RN
FROM SomeDbSet s
WHERE ...
) r
WHERE r.RN = 1
For those who want to stay with LINQ, I propose extension (disclaimer: I'm extension creator) linq2db.EntityFrameworkCore
And you can write query above via LINQ
var rnQuery =
from s in context.SomeDbSet
where ...
select new
{
Data = s,
RN = Sql.Ext.RowNumber().Over()
.PartitionBy(s.SomeField1, s.SomeField2)
.OrderBy(s.SomeDate)
.ToValue()
}
var resultQuery = await rnQuery
.Where(r => r.RN == 1)
.Select(r => r.Data)
.ToLinqToDB();
var result = resultQuery.ToList();
// async variant may need ToListAsyncLinqToDB() call
// because of collision in async extension methods between EF Core and linq2db
var result = await resultQuery.ToListAsyncLinqToDB();