C# / LINQ: How to GroupBy when a column's value is not constant

Question

The snippet below shows the method I am using to (try to) pick the "latest" driver of a vehicle when they have the same VIN. This method is called AFTER I use LINQ to get all transactions for all vehicles at a certain location. In the

group vehicle by new

section, DriverLastName is the only column that can change from row to row. The return type of vehiclesGrp is IQueryable

 var vehiclesGrp = from vehicle in inVehicles group vehicle by new { vehicle.CCName, vehicle.DADDivision, vehicle.DADArea, vehicle.DADDistrict, vehicle.DADCity, vehicle.Vin, vehicle.CostCenter, vehicle.GLDivision, vehicle.VehicleMake, vehicle.FuelTankCapacity, vehicle.FuelType, vehicle.EplanNumber, vehicle.LicensePlate, vehicle.DateInService, vehicle.EstimatedMpg, vehicle.VehicleStatus, vehicle.DriverLastName, } into grp select new VehicleDetail { Account = "", ActualMpg = 0, NoteID = 0, NoteCount = 0, CardNum = "", CCName = grp.Key.CCName, CostCenter = grp.Key.CostCenter, DADArea = grp.Key.DADArea, DADCity = grp.Key.DADCity, DADDistrict = grp.Key.DADDistrict, DADDivision = grp.Key.DADDivision, DateInService = grp.Key.DateInService, DriverLastName = grp.Key.DriverLastName, EplanNumber = grp.Key.EplanNumber, EstimatedMpg = grp.Key.EstimatedMpg, Exception = "", Exceptions = grp.Sum(vehicle => vehicle.Exception == null ? 0 : 1), ExpenseType = "", FuelGals = grp.Sum(vehicle => vehicle.FuelGals), FuelPin = "", FuelTankCapacity = grp.Key.FuelTankCapacity, FuelType = grp.Key.FuelType, FuelUnitCost = 0, GLDivision = grp.Key.GLDivision, IntAcctPd = 0, InvoiceLineItem = "", LicensePlate = grp.Key.LicensePlate, MaintenanceKey = "", Odometer = grp.Max(vehicle => vehicle.Odometer), OutOfServiceDate = null, SwdsStoreCc = "", TotalAllCost = grp.Sum(vehicle => vehicle.TotalAmount), TotalAmount = 0, TotalFuelCost = grp.Sum(vehicle => vehicle.ExpenseType == "FUEL" ? vehicle.TotalAmount : 0), TotalMaintCost = grp.Sum(vehicle => vehicle.ExpenseType == "MAINT" ? vehicle.TotalAmount : 0), TotalMiscCost = grp.Sum(vehicle => vehicle.ExpenseType == "MISC" ? vehicle.TotalAmount : 0), TransactionDate = grp.Max(vehicle => vehicle.TransactionDate), TranTime = "", TranDay = "", UnitCost = 0, VehicleMake = grp.Key.VehicleMake, VehicleStatus = grp.Key.VehicleStatus, VendorAddress = "", VendorCity = "", VendorName = "", VendorState = "", VendorZip = "", Vin = grp.Key.Vin }; return vehiclesGrp; 

If driver "Mike" has transactions on the 31st of the month, and driver "John" does not, then Mike should show up as the driver. If they both show up on the 31st, I don't care which driver, as long as the VIN is not duplicated.

Any help on this will be greatly appreciated.

Also, if there is any info I left out, let me know, I'll provide what I can.

Answer 1

I ended up changing the grouping from what I originally posted to

var vehiclesGrp = (from v in inVehicles.AsParallel().AsQueryable()
                  orderby v.TransactionDate descending 
                     group v by new
                                {
                                 v.CCName,
                                 v.SwdsStoreCc,
                                 v.DADDivision,
                                 v.DADArea,
                                 v.DADDistrict,
                                 v.DADCity,
                                 v.Vin,
                                 v.CostCenter,                                     
                                 v.GLDivision,
                                 v.VehicleMake,
                                 v.FuelTankCapacity,
                                 v.FuelType,
                                 v.EplanNumber,
                                 v.LicensePlate,
                                 v.DateInService,
                                 v.EstimatedMpg,                                    
                                 v.VehicleStatus,                                      
                                }
                      into gp
                      let uniqueVinAnyDriver = gp.Max(v => v.DriverLastName)
                      select new VehicleDetail

which provided exactly what I needed in terms of returning only the "latest" driver of the vehicle.