Question

我很难理解LINQ语法以及如何在数据库中使用它与多对多关系。在我的示例项目中，我正在为汽车建模汽车和维护。所以我有3张桌子：

汽车（有关用户添加的汽车的信息）
服务（仅供参考）
CarServices（存储已在汽车上完成的服务）

在网站上，我显示了所有这3个表中的信息，因此我创建了一个视图模型CarsIndexViewModel来保存Car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; }
}

我已经能够在我的控制器中使用此代码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);

我无法弄清楚的是如何只返回符合特定条件的CarServices。例如，如果我只想从最近的日期显示CarServices怎么办？

Answer 1

如果您想展示最新服务：

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。获得你所谈论的几种可能性（从最近的日期开始显示CarServices）是这样的：

上次n近期服务：

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();

指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();

所有服务最后一天都发生的最近服务：

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();

这完全取决于你要做的事情。

在C＃asp.net中使用LINQ返回多对多结果

2 个答案: