asp.net-mvc 如何从 EF 的表中只选择一些字段

How to select just some fields from a table in EF

提问by ravy amiry

I have a table with 9 columns in database and I want to be able to load only some fields of it if I need.

我在数据库中有一个包含 9 列的表，如果需要，我希望能够仅加载其中的某些字段。

How can I do this with Entity Framework 4 please?

请问如何使用 Entity Framework 4 做到这一点？

e.g. My table has these fields:

例如我的表有这些字段：

ID, FirstName, LastName, FotherName, BirthDate, Mobile, Email

and I want to be able to fetch just these columns:

我希望能够只获取这些列：

ID, FirstName, LastName

My project is an ASP.NET MVC 3application, with SQLServer 2008 Expressand EF 4.1.

我的项目是一个ASP.NET MVC 3应用程序，带有SQLServer 2008 Express和EF 4.1。

回答by ravy amiry

Assume you have a table with this model:

假设您有一个具有此模型的表：

public class User{
    public int ID {get; set;}
    public string NickName {get; set;}
    public string FirstName {get; set;}
    public string LastName {get; set;}
    public string FotherName {get; set;}
    public DateTime BirthDate {get; set;}
    public string Mobile {get; set;}
    public string Email {get; set;}
    public string Password {get; set;}
}

Now, you want fetch just ID, FirstName, LastName, and FotherName. You can do it in 2 way; The first way is fetch them as an anonymousobject, look:

现在，你要取刚ID，FirstName，LastName，和FotherName。您可以通过 2 种方式进行操作；第一种方法是将它们作为一个anonymous对象来获取，看看：

var user = entityContext.Users.Where(u => u.ID == id)
    .Select(u => new {
        ID = u.ID,
        FirstName = u.FirstName,
        LastName = u.LastName,
        FotherName = u.FotherName
    }).Single();

Now, your return-value-type is anonymous, you can work with it such as:

现在，您的返回值类型是anonymous，您可以使用它，例如：

var i = user.ID;
// or
var s = user.FirstName;

In another way (for example when you want to passthe object as an Modelto a View), you can define a new class, (i.e. UserViewModel), and when you select the object, select it as a UserViewModel. look:

以另一种方式（例如，当你想传递的对象作为模型的视图），可以定义一个新的类，（即UserViewModel），当您选择的对象，选择它作为一个UserViewModel。看：

public class UserViewModel{
    public int ID {get; set;}
    public string NickName {get; set;}
    public string FirstName {get; set;}
    public string LastName {get; set;}
    public string FotherName {get; set;}
}

and in query, take this:

并在查询中，采取这个：

var user = entityContext.Users.Where(u => u.ID == id)
    .Select(u => new UserViewModel {
        ID = u.ID,
        FirstName = u.FirstName,
        LastName = u.LastName,
        FotherName = u.FotherName
    }).Single();

Look that just ONEdifference is between them, in labda expression, instead of u => new {}we are using u => new UserViewModel{}. Good luck.

看看它们之间只有一个区别，在labda 表达式中，而不是u => new {}我们使用u => new UserViewModel{}. 祝你好运。

回答by Rahmat Ali

There can be many ways to do this job, but using AutomapperNuGet package is the most simple one I have experienced.

有很多方法可以完成这项工作，但使用AutomapperNuGet 包是我体验过的最简单的一种。

• First: Install AutmapperNuGet package for your project from NuGet package explorer.

• Second: Make a simple ViewModel, which contains only required attributes:

  public class UserViewModel {
      public int ID {get; set;}
      public string FirstName {get; set;}
      public string LastName {get; set;}
  }
  

• Third: Initialize your your mapper only for once in app_startclass like:

  namespace SampleProject.App_Start {
      public class AutoMapperConfig {
          public static void Initializer() {
              AutoMapper.Mapper.Initialize(cfg => {
  
                  cfg.CreateMap<User, UserViewModel>()
              });
           }
       }
  }
  

• Fourth: Add it's entry in Global.asax.cs:

  namespace SampleProject
  {
      public class MvcApplication : System.Web.HttpApplication
      {
          protected void Application_Start()
          {
              AreaRegistration.RegisterAllAreas();
              GlobalConfiguration.Configure(WebApiConfig.Register);
              FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
              RouteConfig.RegisterRoutes(RouteTable.Routes);
              BundleConfig.RegisterBundles(BundleTable.Bundles);
  
              // AutoMapper Initializer
              App_Start.AutoMapperConfig.Initializer();
          }
      }
  }
  

• Fifth: Use it in your controller where you want like this:

  namespace SampleProject.Controllers
  {
      public class UsersController : Controller
      {
          private DataContext db = new DataContext();
  
          public ActionResult Index()(
              var model = AutoMapper.Mapper.Map<List<UserViewModel>>(db.User.ToList());
              return View(model);
  
          }
      }
  }
  

• Last: You can create as many maps as you want between your Modelsand ViewModelsby initializing them once in the app_start's AutoMapperConfigclass and use them where you want with a simple line of code.

• 首先：Autmapper从 NuGet 包资源管理器为您的项目安装NuGet 包。

• 第二：做一个简单的ViewModel，只包含必需的属性：

  public class UserViewModel {
      public int ID {get; set;}
      public string FirstName {get; set;}
      public string LastName {get; set;}
  }
  

• 第三：在app_start课堂上只初始化一次你的映射器，比如：

  namespace SampleProject.App_Start {
      public class AutoMapperConfig {
          public static void Initializer() {
              AutoMapper.Mapper.Initialize(cfg => {
  
                  cfg.CreateMap<User, UserViewModel>()
              });
           }
       }
  }
  

• 第四：添加它的条目Global.asax.cs：

  namespace SampleProject
  {
      public class MvcApplication : System.Web.HttpApplication
      {
          protected void Application_Start()
          {
              AreaRegistration.RegisterAllAreas();
              GlobalConfiguration.Configure(WebApiConfig.Register);
              FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
              RouteConfig.RegisterRoutes(RouteTable.Routes);
              BundleConfig.RegisterBundles(BundleTable.Bundles);
  
              // AutoMapper Initializer
              App_Start.AutoMapperConfig.Initializer();
          }
      }
  }
  

• 第五：在您想要的控制器中使用它，如下所示：

  namespace SampleProject.Controllers
  {
      public class UsersController : Controller
      {
          private DataContext db = new DataContext();
  
          public ActionResult Index()(
              var model = AutoMapper.Mapper.Map<List<UserViewModel>>(db.User.ToList());
              return View(model);
  
          }
      }
  }
  

• 最后：您可以在Models和之间创建任意数量的映射，方法ViewModels是在app_start'sAutoMapperConfig类中初始化它们，并通过简单的一行代码在需要的地方使用它们。

Also you can find a lot of help about Automapperif you search about it. Its main website is here.

Automapper如果你搜索它，你也可以找到很多帮助。它的主要网站在这里。

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Important:

重要的：

I am a developer of ASP.NET MVC 5. Automapperworks fine for me every time. I cannot check it on MVC 3or older than MVC 5.

我是ASP.NET MVC 5. Automapper每次都对我有用。我无法在MVC 3或 之前检查它MVC 5。