Question

我正在研究ASP.NET MVC项目。我正在使用EF代码优先方法。我有3个班级，分别是：

public class A
{
    public int AID {get;set;}
    public string A1 {get;set}
    public string A2 {get;set}

    public virtual List<B> Bs {get;set;}
}

public class B
{
    public int BID {get;set;}
    public string B1 {get;set}
    public string B2 {get;set}

    public AID {get;set}
    public virtual A A {get;set}

    public virtual List<C> Cs {get;set;}
}

public class C
{
    public int CID {get;set;}
    public string C1 {get;set}
    public string C2 {get;set}

    public BID {get;set}
    public virtual B B {get;set}
}

我想基于B类选择C类的C1属性，其中A1 = 4。我尝试使用：

var result = db.C.select(x=>x.C1).Include(x=>B).where(x=>x.A.equals(4))

我很困惑，不知道如何执行linq查询。此外，我不确定是继续使用预先加载还是继续使用其他内容。

请任何大师请帮帮我吗？

Answer 1

试试这个：

var result = db.C
    .Where(c => c.B.A.A1 == 4)
    .Select(c => c.C1)
    .ToList()

您不必在此处使用Eager loading（Include），因为结果中不包含任何嵌套实体。

预先加载用于解决SELECT N + 1问题。当您检索父实体并想要遍历其子项时，会出现此问题。这导致向数据库发出N + 1个请求。

以下是代码示例：

没有预先加载

var carList = db.Cars.ToList(); //this will create one request to the database to retrieve all cars

foreach(var car in carList)
{
    foreach(var wheel in car.Wheels) //this line will create another request to the database to retrieve wheels for specific car
    }
        Console.Write("Car = {0}, Wheel = {1}", car, wheel);
    }
}
//Total requests: 1 to get car list + N requests to retrieve wheels where N - total number of cars

急切加载

var carList = db.Cars.Include(x => x.Wheels).ToList(); //this will create one request to the database to retrieve all cars together with information about wheels

foreach(var car in carList)
{
    foreach(var wheel in car.Wheels) //this line won't create extra request to the database because this data has been already loaded using eager loading
    }
        Console.Write("Car = {0}, Wheel = {1}", car, wheel);
    }
}
//Total requests: 1 to get car list with all wheel information

Answer 2

默认情况下，EF使用延迟加载，这意味着在你要求之前你不会得到它。使用ToList（）或ToArray（）使EF对sql执行查询并将这些对象放入内存中实体。如果需要，您可以执行原始SQL查询，如YourDbContext.SqlQuery<T>("select * from ...")

渴望加载实体框架，LINQ

2 个答案: