c# - LINQ to Entities 外键关联

Question

我是 asp.net、C# 和 sql 的新手，需要一些指导。我正在使用 :below: db 和 linq-to-entities 框架。我需要将特定的“乘车”与特定的“车辆”相关联，但不确定具体如何进行。我已经在两个对象之间设置了一个导航属性，但现在需要让车辆保存对它所乘坐的游乐设施列表的引用。我是否需要在 vehicle 中单独一列来保存此游乐设施列表？有人可以告诉我完成此操作的语法吗？

这是我目前的代码，在我需要帮助的两个地方有注释:

private void setNewRide(Ride newRide, int carNum)
        {
            using (myEntities = new RamRideOpsEntities())
            {
                var assignedCar = (from car in myEntities.Vehicles
                                where (car.CarNum == carNum)
                                select car).FirstOrDefault();

                if (assignedCar != null && newRide != null)
                {                

                    Ride lastRide = assignedCar.Rides.LastOrDefault(); //HERE IS WHERE I NEED TO LOAD THE MOST RECENT 'RIDE' FOR THIS CAR, IS THIS CORRECT???

                    if (lastRide != null)
                    {
                        lastRide.Status = "Completed";
                        assignedCar.numRides = assignedCar.numRides + 1;
                        lastRide.TimeDroppedOff = DateTime.Now;
                        TimeSpan duration = (lastRide.TimeDroppedOff - lastRide.TimeDispatched).Value;

                        lastRide.ServiceTime = duration;
                        if (assignedCar.AvgRideTime == null)
                        {
                            assignedCar.AvgRideTime = duration;
                        }
                        else
                        {
                            assignedCar.AvgRideTime = TimeSpan.FromSeconds(( ((TimeSpan)assignedCar.AvgRideTime).Seconds + duration.Seconds) / assignedCar.numRides);
                        }
                    }

                    assignedCar.Status = "EnRoute";
                    assignedCar.CurrPassengers = newRide.NumPatrons;
                    assignedCar.StartAdd = newRide.PickupAddress;
                    assignedCar.EndAdd = newRide.DropoffAddress;

                    //HERE IS WHERE I NEED TO ADD THE 'newRide' OBJECT TO THE LIST OF RIDES IN THE 'assignedCar' ..SYNTAX???

                    newRide.TimeDispatched = DateTime.Now;
                    newRide.AssignedCar = carNum;
                    newRide.Status = "EnRoute";

                    myEntities.SaveChanges();
                    SelectCarUP.DataBind();
                    SelectCarUP.Update();                    
                }
            }
        }

Answer 1

对于:

// HERE IS WHERE I NEED TO LOAD THE MOST RECENT 'RIDE' FOR THIS CAR, IS 
// THIS CORRECT???
Ride lastRide = assignedCar.Rides.LastOrDefault();

使用:

Ride lastRide = assignedCar.Rides
    .OrderByDescending(r => r.TimeDispatched)
    .FirstOrDefault();

...以及:

// HERE IS WHERE I NEED TO ADD THE 'newRide' OBJECT TO THE LIST OF RIDES 
// IN THE 'assignedCar' ..SYNTAX???

...根据您的模型图，您的 Vehicle 实体已经有一个 Rides 集合属性，因此您应该能够说:

assignedCar.Rides.Add(newRide);

最后 - 纯粹是个人品味问题 - 而不是:

var assignedCar = (from car in myEntities.Vehicles
                   where (car.CarNum == carNum)
                   select car).FirstOrDefault();

我会使用:

var assignedCar = myEntities.Vehicles
    .FirstOrDefault(car => car.CarNum == carNum);