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

Posted 2020-06-08

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