In the previous part we have determined that:

1. IQueryably consists of a Provider and an Expression Tree
2. Expression Trees can be combined almost as easily as pieces of C# code

In this article, we will look at treating our queries are reusable chunks of logic and combining them into more complex yet still readable queries like this:

public IQueryable<ProductModelOrderStatisticsDto> GetProductModelOrderStats()
{
    // a bigger, more detailed query
    IQueryable<WorkOrderSummaryDto> allDurationsAndRoutings = 
                                                    GetWorkOrderSummaries();

    // is wrapped by an aggregation to retrieve statistics
    var averagePerModel = allDurationsAndRoutings
                .GroupBy(x => new { x.ProductModelId, x.ModelName })
                .Select(x => new ProductModelOrderStatisticsDto
                {
                    ModelId = x.Key.ProductModelId,
                    ModelName = x.Key.ModelName,
                    AverageDuration = x.Where(y => y.DurationDays.HasValue)
                                        .Average(y => y.DurationDays.Value),
                    AverageRoutings = x.Average(y => y.RoutingsCount)
                });

    return averagePerModel;
}