MVC3+EF4.1学习系列(五)----- EF查找导航属性的几种方式

 

文章索引和简介

通过上一篇的学习 我们把demo的各种关系终于搭建里起来 以及处理好了如何映射到数据库等问题 但是 只是搭建好了关系 问题还远没有解决

这篇就来写如何查找导航属性 和查找导航属性的几种方式 已经跟踪生成的SQL来检测是否满意 通过这节学习 来明白什么时候用哪个~~

一.三种加载

1.延迟加载

这是原文中的图 大家可以去看下  我模仿上面的做了个测试  出现了  已有打开的与此 Command 相关联的 DataReader，必须首先将它关闭。

我的解决办法是    var departments = db.Departments.ToList();    现读取出来 然后再遍历. 而不加ToList()  真正执行SQL语句在 foreach的时候

然后再说下 这样写以后 SQL语句的执行

1.上来先查询出所有的Department

SELECT  [Extent1].[DepartmentID] AS [DepartmentID],  [Extent1].[Name] AS [Name],  [Extent1].[Budget] AS [Budget],  [Extent1].[StartDate] AS [StartDate],  [Extent1].[InstructorID] AS [InstructorID] FROM [dbo].[Department] AS [Extent1]

2.再执行到内层foreach时  这个会执行多次  每次@EntityKeyValue1 等于 迭代到这次的 DepartmentID

exec sp_executesql N'SELECT  [Extent1].[CourseID] AS [CourseID],  [Extent1].[Title] AS [Title],  [Extent1].[Credits] AS [Credits],  [Extent1].[DepartmentID] AS [DepartmentID] FROM [dbo].[Course] AS [Extent1] WHERE [Extent1].[DepartmentID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=1

也就是说 我们有多少条Department 就要执行多少次上面的方法   当然 这里使用的是exec sp_executesql   利用sp_executesql，能够重用执行计划，这就大大提供了执行性能

2.贪婪加载

在执行到第一个foreach 时  就执行了SQL语句 这是EF帮我们生成的

SELECT  [Project1].[DepartmentID] AS [DepartmentID],  [Project1].[Name] AS [Name],  [Project1].[Budget] AS [Budget],  [Project1].[StartDate] AS [StartDate],  [Project1].[InstructorID] AS [InstructorID],  [Project1].[C1] AS [C1],  [Project1].[CourseID] AS [CourseID],  [Project1].[Title] AS [Title],  [Project1].[Credits] AS [Credits],  [Project1].[DepartmentID1] AS [DepartmentID1] FROM ( SELECT      [Extent1].[DepartmentID] AS [DepartmentID],      [Extent1].[Name] AS [Name],      [Extent1].[Budget] AS [Budget],      [Extent1].[StartDate] AS [StartDate],      [Extent1].[InstructorID] AS [InstructorID],      [Extent2].[CourseID] AS [CourseID],      [Extent2].[Title] AS [Title],      [Extent2].[Credits] AS [Credits],      [Extent2].[DepartmentID] AS [DepartmentID1],      CASE WHEN ([Extent2].[CourseID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]     FROM  [dbo].[Department] AS [Extent1]     LEFT OUTER JOIN [dbo].[Course] AS [Extent2] ON [Extent1].[DepartmentID] = [Extent2].[DepartmentID] )  AS [Project1] ORDER BY [Project1].[DepartmentID] ASC, [Project1].[C1] ASC

3.显示加载

先看图

这个我测试后 效果是和第一个一样的 并没有看出什么好处？ 期待高手指点下 

英文好的也可以看下原文 

4.关闭延迟加载

如果我们想启用延迟加载 可以通过这两种方式

1.去掉属性里的virtual

2.context.Configuration.LazyLoadingEnabled = false;

二.实战开始 创建教师页

先上实现后的效果图

从图中 我们可以看出这个要处理的关系

1对1的 教师和办公地点

1对多的 教师教的课程

普通的多对多的

多对多的(关系表里有数据的)  课程和学生  查看选择课程的学生和学分

1.创建viewmodel

有时 我们的页面 显示的不是一个实体类的内容  这个时候我们可以创建一个ViewModel 来展示界面

using System; using System.Collections.Generic; using ContosoUniversity.Models;
namespace ContosoUniversity.ViewModels {     public class InstructorIndexData     {         public IEnumerable<Instructor> Instructors { get; set; }         public IEnumerable<Course> Courses { get; set; }         public IEnumerable<Enrollment> Enrollments { get; set; }     } }

2.创建控制器添加Index

public ActionResult Index(Int32? id, Int32? courseID) {     var viewModel = new InstructorIndexData();     viewModel.Instructors = db.Instructors         .Include(i => i.OfficeAssignment)         .Include(i => i.Courses.Select(c => c.Department))         .OrderBy(i => i.LastName);
    if (id != null)     {         ViewBag.InstructorID = id.Value;         viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id.Value).Single().Courses;     }
    if (courseID != null)     {         ViewBag.CourseID = courseID.Value;         viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Single().Enrollments;     }
    return View(viewModel); }

先看进来访问的这一块

  viewModel.Instructors = db.Instructors         .Include(i => i.OfficeAssignment)         .Include(i => i.Courses.Select(c => c.Department))         .OrderBy(i => i.LastName);

从最上面的图中 我们可以看到  要显示有教师信息 办公地址 和所教课程

于是 我们使用贪婪加载出办公地址和课程  但是 原文教程里 还Select(c => c.Department) 把院系也一起加载了进来  我认为这是没必要的

于是 我把代码修改为

 db.Instructors                 .Include(i => i.OfficeAssignment)                 .Include(i => i.Courses)                 .OrderBy(i => i.LastName);

去掉了对院系的贪婪加载

看下生成的SQL语句

SELECT  [Project1].[InstructorID1] AS [InstructorID],  [Project1].[InstructorID] AS [InstructorID1],  [Project1].[LastName] AS [LastName],  [Project1].[FirstName] AS [FirstName],  [Project1].[HireDate] AS [HireDate],  [Project1].[InstructorID2] AS [InstructorID2],  [Project1].[Location] AS [Location],  [Project1].[C1] AS [C1],  [Project1].[CourseID] AS [CourseID],  [Project1].[Title] AS [Title],  [Project1].[Credits] AS [Credits],  [Project1].[DepartmentID] AS [DepartmentID] FROM ( SELECT      [Extent1].[InstructorID] AS [InstructorID],      [Extent1].[LastName] AS [LastName],      [Extent1].[FirstName] AS [FirstName],      [Extent1].[HireDate] AS [HireDate],      [Extent2].[InstructorID] AS [InstructorID1],      [Extent3].[InstructorID] AS [InstructorID2],      [Extent3].[Location] AS [Location],      [Join3].[CourseID1] AS [CourseID],      [Join3].[Title] AS [Title],      [Join3].[Credits] AS [Credits],      [Join3].[DepartmentID] AS [DepartmentID],      CASE WHEN ([Join3].[CourseID2] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]     FROM    [dbo].[Instructor] AS [Extent1]     LEFT OUTER JOIN [dbo].[OfficeAssignment] AS [Extent2] ON [Extent1].[InstructorID] = [Extent2].[InstructorID]     LEFT OUTER JOIN [dbo].[OfficeAssignment] AS [Extent3] ON [Extent2].[InstructorID] = [Extent3].[InstructorID]     LEFT OUTER JOIN  (SELECT [Extent4].[CourseID] AS [CourseID2], [Extent4].[InstructorID] AS [InstructorID], [Extent5].[CourseID] AS [CourseID1], [Extent5].[Title] AS [Title], [Extent5].[Credits] AS [Credits], [Extent5].[DepartmentID] AS [DepartmentID]         FROM  [dbo].[CourseInstructor] AS [Extent4]         INNER JOIN [dbo].[Course] AS [Extent5] ON [Extent5].[CourseID] = [Extent4].[CourseID] ) AS [Join3] ON [Extent1].[InstructorID] = [Join3].[InstructorID] )  AS [Project1] ORDER BY [Project1].[LastName] ASC, [Project1].[InstructorID1] ASC, [Project1].[InstructorID] ASC, [Project1].[InstructorID2] ASC, [Project1].[C1] ASC

继续分析

  if (id != null)     {         ViewBag.InstructorID = id.Value;         viewModel.Courses = viewModel.Instructors.Where(i => i.InstructorID == id.Value).Single().Courses;     }

如果点击教师 则可查看该教师教的课程  这个id 就是教师ID 一会儿会在视图展示这个 这个就是根据教师查看课程 

接着是点击课程 查看所选的学生和分数

    if (courseId != null)             {                 viewModel.Enrollments = viewModel.Courses.Where(i => i.CourseID == courseId.Value).Single().Enrollments;             }

这里还给出里另一种方法

    if (courseID != null)     {         ViewBag.CourseID = courseID.Value;
        var selectedCourse = viewModel.Courses.Where(x => x.CourseID == courseID).Single();         db.Entry(selectedCourse).Collection(x => x.Enrollments).Load();         foreach (Enrollment enrollment in selectedCourse.Enrollments)         {             db.Entry(enrollment).Reference(x => x.Student).Load();         }                                  viewModel.Enrollments = viewModel.Courses.Where(x => x.CourseID == courseID).Single().Enrollments;     }

最后上视图

@model ContosoUniversity.ViewModels.InstructorIndexData
@{     ViewBag.Title = "Instructors"; }
<h2>Instructors</h2>
<p>     @Html.ActionLink("Create New", "Create") </p> <table>      <tr>          <th></th>          <th>Last Name</th>          <th>First Name</th>          <th>Hire Date</th>          <th>Office</th>         <th>Courses</th>     </tr>      @foreach (var item in Model.Instructors)      {          string selectedRow = "";         if (item.InstructorID == ViewBag.PersonID)          {              selectedRow = "selectedrow";          }          <tr class="@selectedRow" valign="top">              <td>                  @Html.ActionLink("Select", "Index", new { id = item.InstructorID }) |                  @Html.ActionLink("Edit", "Edit", new { id = item.InstructorID }) |                  @Html.ActionLink("Details", "Details", new { id = item.InstructorID }) |                  @Html.ActionLink("Delete", "Delete", new { id = item.InstructorID })              </td>              <td>                  @item.LastName              </td>              <td>                  @item.FirstMidName              </td>              <td>                  @String.Format("{0:d}", item.HireDate)              </td>              <td>                  @if (item.OfficeAssignment != null)                  {                      @item.OfficeAssignment.Location                   }              </td>              <td>                 @{                     foreach (var course in item.Courses)                     {                         @course.CourseID @:&nbsp; @course.Title <br />                     }                 }             </td>         </tr>      }  </table>
@if (Model.Courses != null)  {      <h3>Courses Taught by Selected Instructor</h3>  <table>      <tr>          <th></th>          <th>ID</th>          <th>Title</th>          <th>Department</th>      </tr>        @foreach (var item in Model.Courses)      {          string selectedRow = "";          if (item.CourseID == ViewBag.CourseID)          {              selectedRow = "selectedrow";          }      <tr class="@selectedRow">          <td>              @Html.ActionLink("Select", "Index", new { courseID = item.CourseID })          </td>          <td>              @item.CourseID          </td>          <td>              @item.Title          </td>          <td>              @item.Department.Name          </td>      </tr>      }    </table>  }
@if (Model.Enrollments != null)  {      <h3>          Students Enrolled in Selected Course</h3>      <table>          <tr>              <th>Name</th>              <th>Grade</th>          </tr>          @foreach (var item in Model.Enrollments)          {              <tr>                  <td>                      @item.Student.FullName                  </td>                  <td>                      @item.Grade                  </td>              </tr>          }      </table>  }

三.上节的一个问题与疑问的提出

再上节的建立关系中 有一个这样的问题  一对多的关系中 是否应该为导航属性 再专门建立一个ID

比如我们可 课程与院系  一个院系可以有多个课程  一个课程只能属于一个院系 那我们是否应该在课程类里 加入院系ID呢

如

    /// <summary>     /// 课程类     /// </summary>     public class Course     {         /// <summary>         /// 课程ID         /// </summary>         [DatabaseGenerated(DatabaseGeneratedOption.None)]         [Display(Name = "Number")]         public int CourseID { get; set; }         /// <summary>         /// 课程名称         /// </summary>         [Required(ErrorMessage = "Title is required.")]         [MaxLength(50)]         public string Title { get; set; }         /// <summary>         /// 学分         /// </summary>         [Required(ErrorMessage = "Number of credits is required.")]         [Range(0, 5, ErrorMessage = "Number of credits must be between 0 and 5.")]         public int Credits { get; set; }
        [Display(Name = "Department")]         public int DepartmentID { get; set; }
         /// <summary>         /// 关系表导航属性  一个课程允许被多次报名等级         /// </summary>         public virtual ICollection<Enrollment> Enrollments { get; set; }         public virtual Department Department { get; set; }         public virtual ICollection<Instructor> Instructors { get; set; }
     }

这里面加了 院系ID  我以前一直觉得没有必要加这个 今天在做这个导航属性查找时 发现一个问题 做个小实验

比如我想得到其中一个课程的ID 如果有院系ID 属性 可以这么写

   var courses = db.Courses.ToList();       int i = courses[0].DepartmentID;

如果没 可以这么写

 int i = courses[0].Department.DepartmentID;

首先 这个都没有用贪婪加载 默认的延迟加载 如果你使用上面的 则不会往数据库里去执行一条根据课程ID查找院系的SQL语句

但你使用下面的 则会往数据库里发送一条查找语句

这点 EF做的是并不好的 在NH里 两种方法 都不会发送  因为在下面那里使用了代理 而EF没有

我想问的是 是我哪操作的不对么？ 造成了这个原因？ 请高手解答下

四.总结

关系的加载就结束了 其实写关系加载的园子中有不少好文章了 我这里写的少了些~~

不过关系的操作还没有结束

下一篇

导航属性的更新等操作