top_5_documents_by_group.linq

void Main()
{
	
	var sampleData = new List<Foo>();
	sampleData.Add(new Foo { DocumentId = 1, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 1, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 1, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 5, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 6, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 5, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 8, AccessGroupId = 1 });
	sampleData.Add(new Foo { DocumentId = 10, AccessGroupId = 1 });	
	
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 2, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 3, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 2 });
	sampleData.Add(new Foo { DocumentId = 4, AccessGroupId = 2 });		
	
	var x = (from entry in sampleData
	 group entry by new { entry.DocumentId, entry.AccessGroupId } into g
	 orderby  g.Key.AccessGroupId, g.Count() descending
	 select new { DocumentId = g.Key.DocumentId, AccessGroupId = g.Key.AccessGroupId, Count = g.Count() }
	);
	
	Console.WriteLine(x);
}

public class Foo {
	public int DocumentId { get; set; }
	public int AccessGroupId { get; set; }
}