在C＃asp.net中使用LINQ返回多對多結果

Question

我正在努力了解LINQ語法以及如何在數據庫中的多對多關系中使用它。 在我的示例項目中，我正在對汽車進行建模和維護，您可以對汽車進行建模。 所以我有3張桌子：

1. 汽車（用戶添加的汽車信息）
2. 服務（僅參考信息）
3. CarServices（存儲已在汽車上完成的服務）

在網站上，我將顯示所有這三個表中的信息，因此我創建了一個視圖模型CarsIndexViewModel來容納Car及其相關的CarServices。

public class Car
{
    public int Id { get; set; }

    [Required]
    [StringLength(40)]
    public string Name { get; set; }

    [Required]
    [Range(1900, 2018)]
    public int Year { get; set; }
    public string Make { get; set; }
    public string Model { get; set; }
    public string Color { get; set; }
    public int Mileage { get; set; }

    public virtual ICollection<CarService> CarServices { get; set; }
}

public class Service
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual ICollection<CarService> CarServices { get; set; }

}

public class CarService
{
    public int Id { get; set; }

    public int CarId { get; set; }

    [Required]
    [Display(Name = "Service")]
    public int ServiceId { get; set; }

    [Required]
    [DataType(DataType.Date)]
    [DisplayFormat(DataFormatString = "{0:MM/dd/yyyy}")]
    [Display(Name = "Date of Service")]
    public DateTime ServiceDate { get; set; }

    [Column(TypeName = "Money")]
    public decimal? Cost { get; set; }

    public bool HasReceipt { get; set; }

    [Required]
    [Display(Name = "Mileage At Time of Service")]
    public int Mileage { get; set; }

    public virtual Car Car { get; set; }
    public virtual Service Service { get; set; }
}

和視圖模型：

public class CarsIndexViewModel
{
    public Car Car { get; set; }
    public IEnumerable<CarService> Services { get; set; }
}

通過在控制器中使用以下代碼，我已經能夠返回所有服務及其信息：

var query = (
            from car in _context.Cars
            select new CarsIndexViewModel()
            {
                Car = car,
                Services = car.CarServices
            }).ToList();

return View(query);

我無法弄清楚的是如何只退還符合特定條件的CarServices。 例如，如果我只想顯示最近的CarServices怎么辦？

Answer 1

如果要顯示最新服務：

var query = (from car in _context.Cars
             select new CarsIndexViewModel()
             {
                 Car = car,
                 Services = car.CarServices
                               .OrderByDescending(cs => cs.ServiceDate)
                               .Take(5) // only 5 most recent
                               .ToList()
             }).ToList();

Answer 2

有很多方法可以做到這一點。 得到您正在談論的內容的幾種可能性（顯示最近日期的CarServices）是：

最近n服務：

var n = 4;
var query = (
        from car in _context.Cars
        select new CarsIndexViewModel()
        {
            Car = car,
            AllServices = car.CarServices,
            RecentServices = car.CarServices.OrderBy(x => ServiceDate).Take(n)
        }).ToList();

給定日期之后的最新服務：

var recentThreshold = new DateTime(2017,01,01);
var query = (
        from car in _context.Cars
        select new CarsIndexViewModel()
        {
            Car = car,
            AllServices = car.CarServices,
            RecentServices = car.CarServices.Where(x => ServiceDate > recentThreshhold)
        }).ToList();

在提供任何服務的最后一天發生的所有最新服務：

var latestServiceDates = (
        from car in _context.Cars
        select new KeyValuePair<int, DateTime>()
        {
            Key = car.id,
            Value = car.CarServices.OrderByDescending(x => ServiceDate).First()
        }).ToList();
var query = (
        from car in _context.Cars
        select new CarsIndexViewModel()
        {
            Car = car,
            AllServices = car.CarServices,
            RecentServices = car.CarServices.Where(x => ServiceDate = latestServiceDates.First(y => y.Key == x.id))
        }).ToList();

這僅取決於您要確切執行的操作。