I want to create an simple aggregation request using linq expression to extract groups and calculate some values.
It works fine except for property of type TimeSpan.

collection.Aggregate()
.Match(x => x.Date >= minimumDate)
.Group(
x => x.UserId,
group => new

{ id = group.Key, Count = group.Count(), Price = group.Sum(x => x.Price), // float : ok Duration = group.Sum(x => x.Duration) // TimeSpan : ko !! }

);

The Sum of Duration property for group does not work. There is no Sum extension method that accept a TimeSpan in System.Linq.Enumerable.
If I change to x => x.Duration.Ticks, the Driver throws an exception with message saying that it does not know translate Ticks.
I tried to create a Sum extension method accepting a TimeSpan. It compiles but yet fails at runtime because the driver does not recognize this method that is not part of the framework.

So how to do. I do not want to used aggregate API with BsonDocument. I want to use linq expression as it allow to abstract the data access layer and it is great for unit tests.