如何使EntityFramework Relation IQueryable？

Question

我们假设我有这些课程:

public class Car
{
  public int CarId { get; set} 
  public virtual ICollection<Door> Doors { get; set} 
}

public class Door
{
   public int DoorId { get; set} 
   public decimal Weight { get; set} 
   public int CarId { get; set} 
}

我想做这样的事情

foreach ( var car in db.Cars )
{
   var x = car.Doors.Min(d => d.Weight);
}

正如我在EFTraceLog中看到的那样,他从Doors中选择了Select*,其中CarId = @ ...并在应用服务器上计算"Min",而不是在db服务器上计算

我有很大的车​​和门桌,所以这个操作持续几分钟.但如果我改变代码

foreach ( var car in db.Cars )
{
   var x = db.Doors.Where(d => d.CarId == car.CarId).Min(d => d.Weight);
}

那几秒钟.

为什么会有这么大的差异以及如何解决它？这里的问题是编写起来要简单得多

var x = car.Doors.Min(d => d.Weight);

然后

var x = db.Doors.Where(d => d.CarId == car.CarId).Min(d => d.Weight);

更新

我们正在使用Entity Framework 5.0

更新2

我试过这些变种,它们很慢

var x = car.Doors.Select(door => door.Weight).Min();
var x = car.Doors.OrderBy(x => x.Weight).Select(x => x.Weight).FirstOrDefault();
var x = car.Doors.OrderBy(x => x.Weight).Select(x => x.Weight).First();
var x = car.Doors.OrderBy(x => x.Weight).FirstOrDefault().Weight;
var x = car.Doors.OrderBy(x => x.Weight).First().Weight;

只有这一个很快

var x = db.Doors.Where(d => d.CarId == car.CarId).Min(d => d.Weight);

更新3

最好的查询产生这个SQL

declare @p__linq__0 Int32 = cast(N'204' as Int32);

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
MIN([Extent1].[Weight]) AS [A1]
FROM [dbo].[Doors] AS [Extent1]
WHERE [Extent1].[CarId] = @p__linq__0
) AS [GroupBy1]

Answer 1

Linq to Entities只支持Min没有额外的投影.这意味着您Min将使用Linq To Objects调用当前(使您的Doors集合具体化 - 因此您在日志中看到的生成的SQL).

来自MSDN:

支持的:

TSource Min<TSource>(this IQueryable<TSource> source)

不支持:

TResult Min<TSource, TResult>(this IQueryable<TSource> source,Expression<Func<TSource, TResult>> selector)

你可以尝试使用OrderBy,Select和FirstOrDefault(即是支持),以达到同样的效果:

var min = car.Doors.OrderBy(x => x.Weight).Select(x => x.Weight).FirstOrDefault();

更新:

显然Entity Framework不支持Lazy-Load with Projection,这意味着无论何时加载引用的实体(Car.Doors在您的情况下)框架Lazy-Load所有数据,您都无法选择Weight要加载的属性(在您的情况下).

这就是两个电话之间差异的原因:

// Accessing 'Doors' thru 'Car' means 'Lazy Load'
car.Doors.Select(x => x.Weight).Min();

但,

// No 'Lazy Load' involved, hence projection is possible
db.Doors.Where(x => x.CarId == carId).Select(x => x.Weight).Min();

您还可以尝试访问Car.Doors和Eager Load Doors:

foreach ( var car in db.Cars.Include(x => x.Doors))
{
   var x = car.Doors.Select(x => x.Weight).Min();
}

(我使用了ServyMin提出的缩短替代方案)