MongoDB .NET Group by with list result of whole objects - mongodb

Question

I was digging into the documentation of MongoDB http://mongodb.github.io/mongo-csharp-driver/2.7/reference/driver/crud/linq/ and I saw, using .NET drive is possible to make group by on the database.
For 1 ProductId I can have few elements in a database. I want to get whole Last element.
So I am trying to do something like:
var query = _collection.AsQueryable()
.GroupBy(p => p.ProductId, (k, s) =>
new { Name = k, Result = s.First() })
.Select(x => new { Name = x.Name, Result = x.Result }).First();
The problem is that I see an error message like:
System.NotSupportedException: Specified method is not supported.
at
MongoDB.Driver.Linq.Processors.AccumulatorBinder.GetAccumulatorArgument(Expression node)
I know that for now in my example i didnt order by the result.. But this will be my second step. For now i see that i cannot group by. Is it possible to do this kind of group by?

Answer 1

My solution for that is:
var query = _collection.Aggregate(new AggregateOptions { AllowDiskUse = true })
.Match(x => ElementIds.Contains(x.ElementId))
.SortByDescending(x => x.StartDate).ThenByDescending(x => x.CreatedAt)
.Group(x => x.ElementId, x => new
{
StartDate = x.First().StartDate,
Grades = x.First().Grades,
SellingId = x.First().SellingId,
CreatedAt = x.First().CreatedAt,
ModifiedAt = x.First().ModifiedAt,
Id = x.First().Id,
ElementId = x.First().ElementId
})
.ToEnumerable(token);
After that, I parsed it into my model.
AllowDiskUse = true, because in my case MongoDB's memory is not enough to handle this operation.