Jeg fandt en løsning, jeg har forberedt ekstra klasse:
[BsonIgnoreExtraElements]
public class UnwindedOrderItem
{
public OrderItem OrderItems { get; set; }
}
var agg = database.GetCollection<Order>("Order")
.Aggregate()
.Unwind<Order, UnwindedOrderItem>(x => x.OrderItems)
.Group(x=>x.OrderItems.ProductId, g => new
{
Id = g.Key,
Suma = g.Sum(x=>x.OrderItems.PriceExclTax)
})
.ToListAsync().Result;