在 C# asp.net 中使用 LINQ 返回多对多结果

Question

我正在努力理解 LINQ 语法以及如何将它与数据库中的多对多关系一起使用。在我的示例项目中，我正在对汽车进行建模，并且您可以对汽车进行维护。所以我有 3 张桌子：

1. 汽车（用户添加的汽车信息）
2. 服务（仅供参考）
3. CarServices（存储在汽车上完成的服务）

在网站上，我显示了来自所有 3 个表的信息，因此我创建了一个视图模型 CarsIndexViewModel 来保存汽车及其关联的 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; }
}

和视图模型：

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

通过在我的控制器中使用此代码，我已经能够返回所有服务及其信息：

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

return View(query);

我不知道如何只返回满足特定条件的 CarServices。例如，如果我只想显示最近日期的 CarServices 怎么办？

Answer 1

如果您想显示最近的服务：

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这一点。了解您正在谈论的内容的几种可能性（显示最近日期的 CarServices）是：

最近n最近的服务：

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();

给定日期之后的最近服务：

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();

最近的服务都发生在提供任何服务的最后一天：

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();

这完全取决于你想要做什么。