How to select top N rows for each group in a Entity Framework GroupBy with EF 3.1

Question

I need to get top 10 rows for each group in a table with entity framework. Based on other solution on SO, I tried 2 things:

var sendDocuments = await context         


        

Answer 1

This is a common problem, unfortunately not supported by EF Core 3.0/3.1 query translator specifically for GroupBy.

The workaround is to do the groping manually by correlating 2 subqueries - one for keys and one for corresponding data.

Applying it to your examples would be something like this.

If you need (key, items) pairs:

var query = context.Set<DbDocument>()
    .Where(t => partnerIds.Contains(t.SenderId))
    .Select(t => t.SenderId).Distinct() // <--
    .Select(key => new
    {
        Key = key,
        Documents = 
            context.Set<DbDocument>().Where(t => t.SenderId == key) // <--
                 .OrderByDescending(t => t.InsertedDateTime).Take(10)
                 .ToList() // <--
    });

If you need just flat result set containing top N items per key:

var query = context.Set<DbDocument>()
    .Where(t => partnerIds.Contains(t.SenderId))
    .Select(t => t.SenderId).Distinct() // <--
    .SelectMany(key => context.Set<DbDocument>().Where(t => t.SenderId == key) // <--
        .OrderByDescending(t => t.InsertedDateTime).Take(10)
    );

Answer 2

maybe try a different approach?

 var partnersList = context.partnerIds.Where(x=> "your where clause here").Take(10).Tolist()