MVC中使用EF(6):更新关联数据
Contoso University示例网站演示如何使用Entity Framework 5创建ASP.NET MVC 4应用程序。Entity Framework有三种处理数据的方式: Database First, Model First, and Code First. 本指南使用代码优先。其它方式请查询资料。示例程序是为Contoso University建立一个网站。功能包括：学生管理、课程创建、教师分配。 本系列指南逐步讲述如何实现这一网站程序。
如有问题，可在这些讨论区提问： ASP.NET Entity Framework forum, the Entity Framework and LINQ to Entities forum, or StackOverflow.com.
上一节完成了相关联数据的显示，本节将学习如何更新关联数据。大部分关联关系可通过更新相应的外键来完成。对于多对多关系，EF没有直接暴漏连接表，需要显式的操作导航属性（向其中添加、移除实体）来完成。
将要完成的效果如下：
课程实体创建后是和某个部门有关联的。为了展示这一点，自动生成的代码生成了相应的控制器方法以及创建、编辑视图，其中包括可选择部门的下拉列表。下拉列表设置 Course.DepartmentID外键属性，这样EF就可以正确加载Department 导航属性的对应实体。这里只简单修改代码，增加错误处理和下拉列表排序功能。
在 CourseController.cs, Edit 和 Create 方法修改后代码如下:
public ActionResult Create(){ PopulateDepartmentsDropDownList(); return View();}[HttpPost][ValidateAntiForgeryToken]public ActionResult Create( [Bind(Include = "CourseID,Title,Credits,DepartmentID")] Course course){ try { if (ModelState.IsValid) { db.Courses.Add(course); db.SaveChanges(); return RedirectToAction("Index"); } } catch (DataException /* dex */) { //Log the error (uncomment dex variable name after DataException and add a line here to write a log.) ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator."); } PopulateDepartmentsDropDownList(course.DepartmentID); return View(course);}public ActionResult Edit(int id){ Course course = db.Courses.Find(id); PopulateDepartmentsDropDownList(course.DepartmentID); return View(course);}[HttpPost][ValidateAntiForgeryToken]public ActionResult Edit( [Bind(Include = "CourseID,Title,Credits,DepartmentID")] Course course){ try { if (ModelState.IsValid) { db.Entry(course).State = EntityState.Modified; db.SaveChanges(); return RedirectToAction("Index"); } } catch (DataException /* dex */) { //Log the error (uncomment dex variable name after DataException and add a line here to write a log.) ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator."); } PopulateDepartmentsDropDownList(course.DepartmentID); return View(course);}private void PopulateDepartmentsDropDownList(object selectedDepartment = null){ var departmentsQuery = from d in db.Departments orderby d.Name select d; ViewBag.DepartmentID = new SelectList(departmentsQuery, "DepartmentID", "Name", selectedDepartment);} PopulateDepartmentsDropDownList 方法获取按名排列的部门列表,为下拉列表构建一个SelectList 集合，使用 ViewBag 属性将其传递到视图.方法有一个可选参数selectedDepartment 以便设置下拉列表默认值.视图将把DepartmentID 传递给DropDownList 帮助器,帮助器从 ViewBag 中寻找名为DepartmentID的 SelectList.
HttpGet Create 调用 PopulateDepartmentsDropDownList 方法时不使用默认值，因为此时还没有创建新课程数据:
public ActionResult Create(){ PopulateDepartmentsDropDownList(); return View();}HttpGet Edit 方法则设置默认值，因为此时课程在编辑时有原始的部门信息:
public ActionResult Edit(int id){ Course course = db.Courses.Find(id); PopulateDepartmentsDropDownList(course.DepartmentID); return View(course);}HttpPost 方法在捕获异常之后再次显示创建或编辑页面时，初始化下拉列表默认值:
catch (DataException /* dex */) { //Log the error (uncomment dex variable name after DataException and add a line here to write a log.) ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator."); } PopulateDepartmentsDropDownList(course.DepartmentID); return View(course);代码确保如果发生异常返回页面时，原有的操作数据还在.
在 Views\Course\Create.cshtml,在 Title 域之前添加代码，提供录入课程编号的编辑域。之前曾经介绍过，自动生成代码不会保护对主键的编辑域.
@model ContosoUniversity.Models.Course@{ ViewBag.Title = "Create";}<h2>Create</h2>@using (Html.BeginForm()){ @Html.AntiForgeryToken() @Html.ValidationSummary(true) <fieldset> <legend>Course</legend> <div class="editor-label"> @Html.LabelFor(model => model.CourseID) </div> <div class="editor-field"> @Html.EditorFor(model => model.CourseID) @Html.ValidationMessageFor(model => model.CourseID) </div> <div class="editor-label"> @Html.LabelFor(model => model.Title) </div> <div class="editor-field"> @Html.EditorFor(model => model.Title) @Html.ValidationMessageFor(model => model.Title) </div> <div class="editor-label"> @Html.LabelFor(model => model.Credits) </div> <div class="editor-field"> @Html.EditorFor(model => model.Credits) @Html.ValidationMessageFor(model => model.Credits) </div> <div class="editor-label"> @Html.LabelFor(model => model.DepartmentID, "Department") </div> <div class="editor-field"> @Html.DropDownList("DepartmentID", String.Empty) @Html.ValidationMessageFor(model => model.DepartmentID) </div> <p> <input type="submit" value="Create" /> </p> </fieldset>}<div> @Html.ActionLink("Back to List", "Index")</div>@section Scripts { @Scripts.Render("~/bundles/jqueryval")}在 Views\Course\Edit.cshtml, Views\Course\Delete.cshtml,和Views\Course\Details.cshtml,添加如下代码
<div class="editor-label"> @Html.LabelFor(model => model.CourseID)</div><div class="editor-field"> @Html.DisplayFor(model => model.CourseID)</div>
运行 Create 页面:
点击Create. Index页面将显示新创建的课程信息。列表中的部门名称来自于导航属性，说明正确建立了关联。
运行 Edit 页面.
修改数据，点击 Save.索引页面显示更新后的数据.
编辑instructor 记录时希望同时更新所在办公室信息. Instructor 和 OfficeAssignment 实体存在一对零或一的关系,也就是要处理以下场景:
在 InstructorController.cs ， 自动生成的HttpGet Edit 方法代码如下:
public ActionResult Edit(int id = 0){ Instructor instructor = db.Instructors.Find(id); if (instructor == null) { return HttpNotFound(); } ViewBag.InstructorID = new SelectList(db.OfficeAssignments, "InstructorID", "Location", instructor.InstructorID); return View(instructor);}自动生成的代码创建了下拉列表，我们将其修改以下，使用文本框：
public ActionResult Edit(int id){ Instructor instructor = db.Instructors .Include(i => i.OfficeAssignment) .Where(i => i.InstructorID == id) .Single(); return View(instructor);}代码通过贪婪加载方式获取OfficeAssignment 实体。Find 方法无法使用贪婪加载，因此使用Where 和Single 方法。
将HttpPost Edit 方法替换为如下代码:
[HttpPost][ValidateAntiForgeryToken]public ActionResult Edit(int id, FormCollection formCollection){ var instructorToUpdate = db.Instructors .Include(i => i.OfficeAssignment) .Where(i => i.InstructorID == id) .Single(); if (TryUpdateModel(instructorToUpdate, "", new string[] { "LastName", "FirstMidName", "HireDate", "OfficeAssignment" })) { try { if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment.Location)) { instructorToUpdate.OfficeAssignment = null; } db.Entry(instructorToUpdate).State = EntityState.Modified; db.SaveChanges(); return RedirectToAction("Index"); } catch (DataException /* dex */) { //Log the error (uncomment dex variable name after DataException and add a line here to write a log. ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists, see your system administrator."); } } ViewBag.InstructorID = new SelectList(db.OfficeAssignments, "InstructorID", "Location", id); return View(instructorToUpdate);}这部分代码的作用是:
从数据库通过贪婪加载获取Instructor 和 OfficeAssignment实体.这是和 HttpGet Edit 方法一样的.
使用模型绑定器数据更新 Instructor 实体. TryUpdateModel 更新白名单中的属性值，关于白名单的介绍在指南的第二节..
if (TryUpdateModel(instructorToUpdate, "", new string[] { "LastName", "FirstMidName", "HireDate", "OfficeAssignment" }))如果办公室信息为空，将 Instructor.OfficeAssignment 属性设为null， OfficeAssignment 表中相应的记录也将删除.
if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment.Location)){ instructorToUpdate.OfficeAssignment = null;}在 Views\Instructor\Edit.cshtml, Hire Date 的div 标记之后,添加办公室信息的编辑域:
<div class="editor-label"> @Html.LabelFor(model => model.OfficeAssignment.Location)</div><div class="editor-field"> @Html.EditorFor(model => model.OfficeAssignment.Location) @Html.ValidationMessageFor(model => model.OfficeAssignment.Location)</div>
运行，测试效果
教师可同时承担多门课程，添加为教师编辑页面增加通过组选框设定承担课程的功能:
Course 和 Instructor 实体之间是多对多关系，意味着无法之间访问连接表。通过 Instructor.Courses导航属性中添加或删除关联实体的方式来实现关系的维护.
功能通过组选框来实现。列出数据库中所有课程，通过选择框确定是否选择，教师当前承担的课程处于选中状态。用户通过选中或者取消选中的操作修改课程的分配情况.如果课程数目很多，你可能希望使用别的显示方法，但操作导航属性来添加或删除关系的方法是一样的。
创建模型类以便为视图的组选框提供数据.在ViewModels 文件夹创建 AssignedCourseData.cs，代码如下:
namespace ContosoUniversity.ViewModels{ public class AssignedCourseData { public int CourseID { get; set; } public string Title { get; set; } public bool Assigned { get; set; } }}在 InstructorController.cs,替换 HttpGet Edit 方法代码如下.
public ActionResult Edit(int id){ Instructor instructor = db.Instructors .Include(i => i.OfficeAssignment) .Include(i => i.Courses) .Where(i => i.InstructorID == id) .Single(); PopulateAssignedCourseData(instructor); return View(instructor);}private void PopulateAssignedCourseData(Instructor instructor){ var allCourses = db.Courses; var instructorCourses = new HashSet<int>(instructor.Courses.Select(c => c.CourseID)); var viewModel = new List<AssignedCourseData>(); foreach (var course in allCourses) { viewModel.Add(new AssignedCourseData { CourseID = course.CourseID, Title = course.Title, Assigned = instructorCourses.Contains(course.CourseID) }); } ViewBag.Courses = viewModel;}代码使用贪婪模式加载 Courses 导航属性，调用PopulateAssignedCourseData 方法实现为视图提供AssignedCourseData 视图模型的数据.
PopulateAssignedCourseData 方法读取所有 Course 实体.对每一个Courses 检查是否已经存在于导航属性.为了提高效率，将当前承担课程的ID形成一个 HashSet 集合.承担课程的 Assigned 属性将设为 true .视图将使用此属性决定哪些选择框处于被选中状态.最后通过ViewBag 的一个属性将列表传递到视图.
下一步，完成保存代码。
使用如下代码替换 HttpPost Edit 方法的代码,调用一个新的方法更新Instructor 实体的 Courses 导航属性.
[HttpPost][ValidateAntiForgeryToken]public ActionResult Edit(int id, FormCollection formCollection, string[] selectedCourses){ var instructorToUpdate = db.Instructors .Include(i => i.OfficeAssignment) .Include(i => i.Courses) .Where(i => i.InstructorID == id) .Single(); if (TryUpdateModel(instructorToUpdate, "", new string[] { "LastName", "FirstMidName", "HireDate", "OfficeAssignment" })) { try { if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment.Location)) { instructorToUpdate.OfficeAssignment = null; } UpdateInstructorCourses(selectedCourses, instructorToUpdate); db.Entry(instructorToUpdate).State = EntityState.Modified; db.SaveChanges(); return RedirectToAction("Index"); } catch (DataException /* dex */) { //Log the error (uncomment dex variable name after DataException and add a line here to write a log. ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists see your system administrator."); } } PopulateAssignedCourseData(instructorToUpdate); return View(instructorToUpdate);}private void UpdateInstructorCourses(string[] selectedCourses, Instructor instructorToUpdate){ if (selectedCourses == null) { instructorToUpdate.Courses = new List<Course>(); return; } var selectedCoursesHS = new HashSet<string>(selectedCourses); var instructorCourses = new HashSet<int> (instructorToUpdate.Courses.Select(c => c.CourseID)); foreach (var course in db.Courses) { if (selectedCoursesHS.Contains(course.CourseID.ToString())) { if (!instructorCourses.Contains(course.CourseID)) { instructorToUpdate.Courses.Add(course); } } else { if (instructorCourses.Contains(course.CourseID)) { instructorToUpdate.Courses.Remove(course); } } }}视图不包含Course 实体集合,因此模型绑定器不能直接更新Courses 导航属性.更新由 UpdateInstructorCourses 方法完成.因此要把 Courses属性从模型绑定器中排除出去.这并不需要修改 TryUpdateModel 的代码，因为使用了白名单， Courses 不在名单之内.
如果没有选中任何课程， UpdateInstructorCourses 将 Courses 导航属性设为一个空的列表:
if (selectedCourses == null){ instructorToUpdate.Courses = new List<Course>(); return;}代码执行循环检查数据库中的每一课程，若此课程被选中则判断是否已经包含在相关数据中，如果没有则添加到导航属性。为了提高效率，把选中课程Id和已有课程ID放在哈希表中。
if (selectedCoursesHS.Contains(course.CourseID.ToString())){ if (!instructorCourses.Contains(course.CourseID)) { instructorToUpdate.Courses.Add(course); }}如果某课程没有选中但存在于 Instructor.Courses 导航属性,则将其从中移除.
else{ if (instructorCourses.Contains(course.CourseID)) { instructorToUpdate.Courses.Remove(course); }}在 Views\Instructor\Edit.cshtml,添加如下高亮代码，在OfficeAssignment 之后增加选中 Courses 的组选框。
@model ContosoUniversity.Models.Instructor@{ ViewBag.Title = "Edit";}<h2>Edit</h2>@using (Html.BeginForm()){ @Html.AntiForgeryToken() @Html.ValidationSummary(true) <fieldset> <legend>Instructor</legend> @Html.HiddenFor(model => model.InstructorID) <div class="editor-label"> @Html.LabelFor(model => model.LastName) </div> <div class="editor-field"> @Html.EditorFor(model => model.LastName) @Html.ValidationMessageFor(model => model.LastName) </div> <div class="editor-label"> @Html.LabelFor(model => model.FirstMidName) </div> <div class="editor-field"> @Html.EditorFor(model => model.FirstMidName) @Html.ValidationMessageFor(model => model.FirstMidName) </div> <div class="editor-label"> @Html.LabelFor(model => model.HireDate) </div> <div class="editor-field"> @Html.EditorFor(model => model.HireDate) @Html.ValidationMessageFor(model => model.HireDate) </div> <div class="editor-label"> @Html.LabelFor(model => model.OfficeAssignment.Location) </div> <div class="editor-field"> @Html.EditorFor(model => model.OfficeAssignment.Location) @Html.ValidationMessageFor(model => model.OfficeAssignment.Location) </div> <div class="editor-field"> <table> <tr> @{ int cnt = 0; List<ContosoUniversity.ViewModels.AssignedCourseData> courses = ViewBag.Courses; foreach (var course in courses) { if (cnt++ % 3 == 0) { @: </tr> <tr> } @: <td> <input type="checkbox" name="selectedCourses" value="@course.CourseID" @(Html.Raw(course.Assigned ? "checked=\"checked\"" : "")) /> @course.CourseID @: @course.Title @:</td> } @: </tr> } </table></div> <p> <input type="submit" value="Save" /> </p> </fieldset>}<div> @Html.ActionLink("Back to List", "Index")</div>@section Scripts { @Scripts.Render("~/bundles/jqueryval")}This code creates an HTML table that hasthree columns. In each column is a check box followed by a caption thatconsists of the course number and title.创建了一个三列表格，每一列包含复选框、课程编号和名称。所有复选框的名字都是一样的("selectedCourses"),模型绑定器由此得知将其作为一组信息来处理.单选框的 value 设为对于课程的 CourseID. 当编辑提交之后，模型绑定器将被选中的复选框的值组合为一个数组传给控制器。
在 Views\Instructor\Index.cshtml,在Office 列之后添加 Courses :
<tr> <th></th> <th>Last Name</th> <th>First Name</th> <th>Hire Date</th> <th>Office</th> <th>Courses</th></tr>
修改视图代码:
@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.InstructorID) { 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 @: @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> @Html.DisplayFor(modelItem => item.Grade) </td> </tr> } </table> }</td>
运行 Instructor Index 查看效果:
点击 Edit 查看Edit page.
修改一些课程的分配然后点击 Save.修改结果在Index页面展示.
Note: 这种方式在课程数目不多时有效。如果课程数目很多需要修改显示方式和更新方法。
更新 Delete 方法
修改代码，当删除教师时，为其分配的办公室信息随之删除:
[HttpPost, ActionName("Delete")][ValidateAntiForgeryToken]public ActionResult DeleteConfirmed(int id){ Instructor instructor = db.Instructors .Include(i => i.OfficeAssignment) .Where(i => i.InstructorID == id) .Single(); instructor.OfficeAssignment = null; db.Instructors.Remove(instructor); db.SaveChanges(); return RedirectToAction("Index");}已经完成了完整的CRUD操作，但没有处理同步问题。下一节将引入同步问题，介绍处理方法，为CRUD操作添加同步处理。
Entity Framework 相关资源，可查看 the last tutorial in this series.