Entity Framework 6 Recipes 2nd Edition（11-4）译 -> 在”模型定义”函数里调用另一个”模型定义”函数

11-4.在”模型定义”函数里调用另一个”模型定义”函数

问题

想要用一个”模型定义”函数去实现另一个”模型定义”函数

解决方案

假设我们已有一个公司合伙人关系连同它们的结构模型,如Figure 11-4所示:

Figure 11-4. A model representing the associate types in a company together with the reporting association

在我们的虚拟的公司里, , team members被一个team leader管理. Team leaders 被 project Managers管理. 而Supervisors管理 project managers.

如果我们想从给定的一个project manager 或supervisor返回所有他们管理的team members ,我们将需要通过  project managers 和team leaders才能获取到 team members. 为了隐藏导航透过这些层的复杂性,我们可以创建”模型定义”函数,来更简单地和直接地访问这些导航属性:

1. 在解决方案资源管理中右击.edmx 文件,打开方式 ➤ XML 编辑器.

2.把Listing 11-7 里的代码插入到概念层的<Schema> 标签下,这样就定义好了函数

Listing 11-7. Model-Defined Functions for Navigating the Associate Hierarchy

<Function Name="GetProjectManager" ReturnType="EFRecipesModel1104.ProjectManager">

<Parameter Name="teammember" Type="EFRecipesModel1104.TeamMember" />

<DefiningExpression>

treat(teammember.Manager.Manager as EFRecipesModel1104.ProjectManager)

</DefiningExpression>

</Function>

<Function Name="GetSupervisor" ReturnType="EFRecipesModel1104.Supervisor">

<Parameter Name="teammember" Type="EFRecipesModel1104.TeamMember" />

<DefiningExpression>

treat(EFRecipesModel1104.GetProjectManager(teammember).Manager as

EFRecipesModel1104.Supervisor)

</DefiningExpression>

</Function>

3.插入和查询模型的代码,如 Listing 11-8所示:

Listing 11-8. Using Both eSQL and LINQ to Query the Model

class Program

{

static void Main(string[] args)

{

RunExample();

Console.WriteLine("\nPress any key to exit...");

Console.ReadKey();

}

static void RunExample()

{

using (var context = new EFRecipesEntities1104())

{

context.Database.ExecuteSqlCommand("delete from chapter11.associate");

var john = new Supervisor { Name = "John Smith" };

var steve = new Supervisor { Name = "Steve Johnson" };

var jill = new ProjectManager

{

Name = "Jill Masterson",

Manager = john

};

var karen = new ProjectManager

{

Name = "Karen Carns",

Manager = steve

};

var bob = new TeamLead { Name = "Bob Richardson", Manager = karen };

var tom = new TeamLead { Name = "Tom Landers", Manager = jill };

var nancy = new TeamMember { Name = "Nancy Jones", Manager = tom };

var stacy = new TeamMember

{

Name = "Stacy Rutgers",

Manager = bob

};

context.Associates.Add(john);

context.Associates.Add(steve);

context.SaveChanges();

}

using (var context = new EFRecipesEntities1104())

{

Console.WriteLine("Using eSQL...");

var emps = context.Associates.OfType<TeamMember>()

//译注:这个where是下载的源码用的形式,但它怎么看也不是eSQL

.Where(q => q.Manager.Name == "Jill Masterson" || q.Manager.Name == "Steve Johnson");

//译注:这个where里书里的代码,倒像eSQL,可是找不到Where这个形式的重载,可能在某个扩展包里,有知晓,望赐教!

//                .Where(@"EFRecipesModel1104.GetProjectManager(it).Name = @projectManager ||

//EFRecipesModel.GetSupervisor(it).Name == @supervisor",

//                new ObjectParameter("projectManager", "Jill Masterson"),

//                new ObjectParameter("supervisor", "Steve Johnson"));

Console.WriteLine("Team members that report up to either");

Console.WriteLine("Project Manager Jill Masterson ");

Console.WriteLine("or Supervisor Steve Johnson");

foreach (var emp in emps)

{

Console.WriteLine("\tAssociate: {0}", emp.Name);

}

}

using (var context = new EFRecipesEntities1104())

{

Console.WriteLine();

Console.WriteLine("Using LINQ...");

var emps = from e in context.Associates.OfType<TeamMember>()

where MyFunctions.GetProjectManager(e).Name ==

"Jill Masterson" ||

MyFunctions.GetSupervisor(e).Name == "Steve Johnson"

select e;

Console.WriteLine("Team members that report up to either");

Console.WriteLine("Project Manager Jill Masterson ");

Console.WriteLine("or Supervisor Steve Johnson");

foreach (var emp in emps)

{

Console.WriteLine("\tAssociate: {0}", emp.Name);

}

}

}

}

public class MyFunctions

{

[EdmFunction("EFRecipesModel1104", "GetProjectManager")]

public static ProjectManager GetProjectManager(TeamMember member)

{

throw new NotSupportedException("Direct calls not supported.");

}

[EdmFunction("EFRecipesModel1104", "GetSupervisor")]

public static Supervisor GetSupervisor(TeamMember member)

{

throw new NotSupportedException("Direct calls not supported.");

}

}

上述Listing 11-8 代码输出如下:

Using eSQL...

Team members that report up to either

Project Manager Jill Masterson

or Supervisor Steve Johnson

Associate: Nancy Jones

Associate: Stacy Rutgers

Using LINQ...

Team members that report up to either

Project Manager Jill Masterson

or Supervisor Steve Johnson

Associate: Nancy Jones

Associate: Stacy Rutgers

它是如何工作的?

在如Listing 11-7所示的GetSupervisor() 函数中,我们需要做给 Manager 属性做三次导航.第一次从TeamMember导航到TeamLead, 第二次从TeamLead,导航到ProjectManager,最后一次从ProjectManager导航到Supervisor. 在Listing 11-7中,我们也定义了GetProjectManager() 函数,所以我们可以用这个函数来简化GetSupervisor() 函数.

我们使用eSQL的treat()运算符把一个公司合伙人实例转化为它们正确的类型 (ProjectManager 或Supervisor). 如果不使用这个运算符,EF将引发一个异常(不能遇到将Associate 映射为ProjectManager 或Supervisor)

在Listing 11-8中, 用 GetProjectManager() 和GetSupervisor() 函数,可以使我们的代码简洁些,因为隐藏了所有通过Manager导航属性穿越对象图的细节

因为我们函数不是返回一个IQueryable<T>, 所以我们不必为LINQ查询实现运行时方法,只需要用到它们的方法存根.

附：创建示例用到的数据库的脚本文件