Custom Aggregate Function

[Siddhant]

I am trying to create and aggregate function

var aggregate = new AggregateFunction<ResourceAllocationBE, double>
{
AggregationExpression = models => models.Sum(model => model.period[0].allocation),
Caption = ""
};

The ResourceAllocationBE contains a period property which is another list that contains allocation and the date. Currently it is taking the first elemnt of the list but i want it to automatically select based on the date alone.
i need to sum up the allocation and that also for a particular date. which might be null some of the times.