Take each first element of the group by

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();