efcore 更新关联表_教程：更新相关数据 - ASP.NET MVC 和 EF Core
教程：更新相关数据 - ASP.NET MVC 和 EF CoreTutorial: Update related data - ASP.NET MVC with EF Core
03/27/2019
本文内容
上一个教程显示出了相关数据，本教程将通过更新外键字段和导航属性来更新相关数据。In the previous tutorial you displayed related data; in this tutorial you'll update related data by updating foreign key fields and navigation properties.
下图是一些将会用到的页面。The following illustrations show some of the pages that you'll work with.
在本教程中，你将了解：In this tutorial, you:
自定义“课程”页Customize Courses pages
添加“讲师编辑”页Add Instructors Edit page
向“编辑”页添加课程Add courses to Edit page
更新“删除”页Update Delete page
向“创建”页添加办公室位置和课程Add office location and courses to Create page
先决条件Prerequisites
自定义“课程”页Customize Courses pages
创建新的课程实体时，新实体必须与现有院系有关系。When a new course entity is created, it must have a relationship to an existing department. 为此，基架代码需包括控制器方法、创建视图和编辑视图，且视图中应包括用于选择院系的下拉列表。To facilitate this, the scaffolded code includes controller methods and Create and Edit views that include a drop-down list for selecting the department. 下拉列表设置了 Course.DepartmentID 外键属性，而这正是 Entity Framework 使用适当的 Department 实体加载 Department 导航属性所需要的。The drop-down list sets the Course.DepartmentID foreign key property, and that's all the Entity Framework needs in order to load the Department navigation property with the appropriate Department entity. 将用到基架代码，但需对其稍作更改，以便添加错误处理和对下拉列表进行排序。You'll use the scaffolded code, but change it slightly to add error handling and sort the drop-down list.
在 CoursesController.cs 中，删除四种 Create 和 Edit 方法，并将其替换为以下代码：In CoursesController.cs, delete the four Create and Edit methods and replace them with the following code:
public IActionResult Create()
{
PopulateDepartmentsDropDownList();
return View();
}[HttpPost]
[ValidateAntiForgeryToken]
public async Task Create([Bind("CourseID,Credits,DepartmentID,Title")] Course course)
{
if (ModelState.IsValid)
{
_context.Add(course);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
PopulateDepartmentsDropDownList(course.DepartmentID);
return View(course);
}public async Task Edit(int? id)
{
if (id == null)
{
return NotFound();
}
var course = await _context.Courses
.AsNoTracking()
.FirstOrDefaultAsync(m => m.CourseID == id);
if (course == null)
{
return NotFound();
}
PopulateDepartmentsDropDownList(course.DepartmentID);
return View(course);
}[HttpPost, ActionName("Edit")]
[ValidateAntiForgeryToken]
public async Task EditPost(int? id)
{
if (id == null)
{
return NotFound();
}
var courseToUpdate = await _context.Courses
.FirstOrDefaultAsync(c => c.CourseID == id);
if (await TryUpdateModelAsync(courseToUpdate,
"",
c => c.Credits, c => c.DepartmentID, c => c.Title))
{
try
{
await _context.SaveChangesAsync();
}
catch (DbUpdateException /* ex */)
{
//Log the error (uncomment ex variable name and write a log.)
ModelState.AddModelError("", "Unable to save changes. " +
"Try again, and if the problem persists, " +
"see your system administrator.");
}
return RedirectToAction(nameof(Index));
}
PopulateDepartmentsDropDownList(courseToUpdate.DepartmentID);
return View(courseToUpdate);
}
在 Edit HttpPost 方法之后，新建一个方法来为下拉列表加载院系信息。After the Edit HttpPost method, create a new method that loads department info for the drop-down list.
private void PopulateDepartmentsDropDownList(object selectedDepartment = null)
{
var departmentsQuery = from d in _context.Departments
orderby d.Name
select d;
ViewBag.DepartmentID = new SelectList(departmentsQuery.AsNoTracking(), "DepartmentID", "Name", selectedDepartment);
}
PopulateDepartmentsDropDownList 方法获取按名称排序的所有院系的列表，为下拉列表创建 SelectList 集合，并将该集合传递给 ViewBag 中的视图。The PopulateDepartmentsDropDownList method gets a list of all departments sorted by name, creates a SelectList collection for a drop-down list, and passes the collection to the view in ViewBag. 该方法可以使用可选的 selectedDepartment 参数，而调用的代码可以通过该参数来指定呈现下拉列表时被选择的项。The method accepts the optional selectedDepartment parameter that allows the calling code to specify the item that will be selected when the drop-down list is rendered. 视图将 DepartmentID 名称传递给 标记帮助器，该帮助器就知道在 ViewBag 对象中查找名为 DepartmentID 的 SelectList。The view will pass the name "DepartmentID" to the tag helper, and the helper then knows to look in the ViewBag object for a SelectList named "DepartmentID".
HttpGet Create 方法调用 PopulateDepartmentsDropDownList 方法，但不会设置选定项，因为对于新课程而言，其院系尚未建立：The HttpGet Create method calls the PopulateDepartmentsDropDownList method without setting the selected item, because for a new course the department isn't established yet:
public IActionResult Create()
{
PopulateDepartmentsDropDownList();
return View();
}
HttpGet Edit 方法根据正在编辑的课程已分配到的院系 ID 设置选定项：The HttpGet Edit method sets the selected item, based on the ID of the department that's already assigned to the course being edited:
public async Task Edit(int? id)
{
if (id == null)
{
return NotFound();
}
var course = await _context.Courses
.AsNoTracking()
.FirstOrDefaultAsync(m => m.CourseID == id);
if (course == null)
{
return NotFound();
}
PopulateDepartmentsDropDownList(course.DepartmentID);
return View(course);
}
Create 和 Edit 这二者的 HttpPost 方法还包括一段代码，用于在错误后重新显示页面时设置选定项。The HttpPost methods for both Create and Edit also include code that sets the selected item when they redisplay the page after an error. 这样可以确保当页面重新显示出现错误消息时，选择的任何院系都将保持选中状态。This ensures that when the page is redisplayed to show the error message, whatever department was selected stays selected.
将 .AsNoTracking 添加到 Details 和 Delete 方法Add .AsNoTracking to Details and Delete methods
为优化“课程详细信息”和“删除”页面的性能，请在 Details 和 HttpGet Delete 方法中添加 AsNoTracking 调用。To optimize performance of the Course Details and Delete pages, add AsNoTracking calls in the Details and HttpGet Delete methods.
public async Task Details(int? id)
{
if (id == null)
{
return NotFound();
}
var course = await _context.Courses
.Include(c => c.Department)
.AsNoTracking()
.FirstOrDefaultAsync(m => m.CourseID == id);
if (course == null)
{
return NotFound();
}
return View(course);
}public async Task Delete(int? id)
{
if (id == null)
{
return NotFound();
}
var course = await _context.Courses
.Include(c => c.Department)
.AsNoTracking()
.FirstOrDefaultAsync(m => m.CourseID == id);
if (course == null)
{
return NotFound();
}
return View(course);
}
修改课程视图Modify the Course views
在 Views/Courses/Create.cshtml 中，向“院系”下拉列表添加一个“选择院系”选项，将标题从 DepartmentID 更改为 Department，并添加一条验证消息 。In Views/Courses/Create.cshtml, add a "Select Department" option to the Department drop-down list, change the caption from DepartmentID to Department, and add a validation message.
-- Select Department --
在 Views/Courses/Edit.cshtml 中，对“院系”字段进行与 Create.cshtml 中相同的更改 。In Views/Courses/Edit.cshtml, make the same change for the Department field that you just did in Create.cshtml.
另外，在 Views/Courses/Edit.cshtml 中，在“标题”字段之前添加一个课程编号字段。Also in Views/Courses/Edit.cshtml, add a course number field before the Title field. 课程编号是主键，因此只会显示，无法更改。Because the course number is the primary key, it's displayed, but it can't be changed.
“编辑”视图中已有一个隐藏的课程编号字段(。There's already a hidden field () for the course number in the Edit view. 添加 标记帮助器后仍然需要该隐藏字段，因为添加标记帮助器后，用户在“编辑”页面上单击“保存”时，已发布数据中并不会包含课程编号 。Adding a tag helper doesn't eliminate the need for the hidden field because it doesn't cause the course number to be included in the posted data when the user clicks Save on the Edit page.
在 Views/Courses/Delete.cshtml 中，在顶部添加一个课程编号字段，并将院系 ID 更改为院系名称。In Views/Courses/Delete.cshtml, add a course number field at the top and change department ID to department name.
@model ContosoUniversity.Models.Course
@{
ViewData["Title"] = "Delete";
}
Delete
Are you sure you want to delete this?
Course
@Html.DisplayNameFor(model => model.CourseID)
@Html.DisplayFor(model => model.CourseID)
@Html.DisplayNameFor(model => model.Title)
@Html.DisplayFor(model => model.Title)
@Html.DisplayNameFor(model => model.Credits)
@Html.DisplayFor(model => model.Credits)
@Html.DisplayNameFor(model => model.Department)
@Html.DisplayFor(model => model.Department.Name)
|
Back to List
在 Views/Courses/Details.cshtml 中，进行对 Delete.cshtml 所作相同的更改 。In Views/Courses/Details.cshtml, make the same change that you just did for Delete.cshtml.
测试“课程”页Test the Course pages
运行应用，选择“课程”选项卡，单击“新建”，然后输入新课程的数据 ：Run the app, select the Courses tab, click Create New, and enter data for a new course:
单击 “创建” 。Click Create. 课程索引页面随即显示，并且新课程已添加在列表中。The Courses Index page is displayed with the new course added to the list. 索引页列表中的院系名称来自导航属性，表明已正确建立关系。The department name in the Index page list comes from the navigation property, showing that the relationship was established correctly.
在课程索引页中的课程上，单击“编辑”。Click Edit on a course in the Courses Index page.
更改页面上的数据，然后单击“保存”。Change data on the page and click Save. 含有更新后的课程数据的“课程索引”页面随即显示。The Courses Index page is displayed with the updated course data.
添加“讲师编辑”页Add Instructors Edit page
编辑讲师记录时，有时希望能更新讲师的办公室分配。When you edit an instructor record, you want to be able to update the instructor's office assignment. Instructor 实体和 OfficeAssignment 实体之间存在一对零或一的关系，这意味着代码必须处理一下情况：The Instructor entity has a one-to-zero-or-one relationship with the OfficeAssignment entity, which means your code has to handle the following situations:
如果用户清除了办公室分配，并且办公室分配最初具有一个值，则删除 OfficeAssignment 实体。If the user clears the office assignment and it originally had a value, delete the OfficeAssignment entity.
如果用户输入了办公室分配值，并且该值最初为空，则创建一个新的 OfficeAssignment 实体。If the user enters an office assignment value and it originally was empty, create a new OfficeAssignment entity.
如果用户更改了办公室分配的值，则更改现有 OfficeAssignment 实体中的值。If the user changes the value of an office assignment, change the value in an existing OfficeAssignment entity.
更新讲师控制器Update the Instructors controller
在 InstructorsController.cs 中，更改 HttpGet Edit 方法中的代码，使其加载 Instructor 实体的 OfficeAssignment 导航属性并调用 AsNoTracking：In InstructorsController.cs, change the code in the HttpGet Edit method so that it loads the Instructor entity's OfficeAssignment navigation property and calls AsNoTracking:
public async Task Edit(int? id)
{
if (id == null)
{
return NotFound();
}
var instructor = await _context.Instructors
.Include(i => i.OfficeAssignment)
.AsNoTracking()
.FirstOrDefaultAsync(m => m.ID == id);
if (instructor == null)
{
return NotFound();
}
return View(instructor);
}
将 HttpPost Edit 方法更新为以下代码，以便处理办公室分配更新：Replace the HttpPost Edit method with the following code to handle office assignment updates:
[HttpPost, ActionName("Edit")]
[ValidateAntiForgeryToken]
public async Task EditPost(int? id)
{
if (id == null)
{
return NotFound();
}
var instructorToUpdate = await _context.Instructors
.Include(i => i.OfficeAssignment)
.FirstOrDefaultAsync(s => s.ID == id);
if (await TryUpdateModelAsync(
instructorToUpdate,
"",
i => i.FirstMidName, i => i.LastName, i => i.HireDate, i => i.OfficeAssignment))
{
if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment?.Location))
{
instructorToUpdate.OfficeAssignment = null;
}
try
{
await _context.SaveChangesAsync();
}
catch (DbUpdateException /* ex */)
{
//Log the error (uncomment ex variable name and write a log.)
ModelState.AddModelError("", "Unable to save changes. " +
"Try again, and if the problem persists, " +
"see your system administrator.");
}
return RedirectToAction(nameof(Index));
}
return View(instructorToUpdate);
}
该代码执行以下操作：The code does the following:
将方法名称更改为 EditPost，因为现在的签名与 HttpGet Edit 方法相同(ActionName 特性指定仍然使用 /Edit/ URL)。Changes the method name to EditPost because the signature is now the same as the HttpGet Edit method (the ActionName attribute specifies that the /Edit/ URL is still used).
使用 OfficeAssignment 导航属性的预先加载从数据库获取当前的 Instructor 实体。Gets the current Instructor entity from the database using eager loading for the OfficeAssignment navigation property. 此操作与在 HttpGet Edit 方法中进行的操作相同。This is the same as what you did in the HttpGet Edit method.
将检索出的 Instructor 实体更新为模型绑定器中的值。Updates the retrieved Instructor entity with values from the model binder. 通过 TryUpdateModel 重载可以声明想包括的属性。The TryUpdateModel overload enables you to declare the properties you want to include. 这样可以防止第二个教程中所述的过度发布。This prevents over-posting, as explained in the second tutorial.
if (await TryUpdateModelAsync(
instructorToUpdate,
"",
i => i.FirstMidName, i => i.LastName, i => i.HireDate, i => i.OfficeAssignment))
如果办公室位置为空，请将 Instructor.OfficeAssignment 属性设置为 NULL，以便删除 OfficeAssignment 表中的相关行。If the office location is blank, sets the Instructor.OfficeAssignment property to null so that the related row in the OfficeAssignment table will be deleted.
if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment?.Location))
{
instructorToUpdate.OfficeAssignment = null;
}
将更改保存到数据库。Saves the changes to the database.
更新讲师编辑视图Update the Instructor Edit view
在 Views/Instructors/Edit.cshtml 中，在“保存”按钮之前的末尾处，添加一个用于编辑办公室位置的新字段：In Views/Instructors/Edit.cshtml, add a new field for editing the office location, at the end before the Save button:
运行应用，选择“讲师”选项卡，然后单击讲师页面上的“编辑” 。Run the app, select the Instructors tab, and then click Edit on an instructor. 更改“办公室位置”，然后单击“保存” 。Change the Office Location and click Save.
向“编辑”页添加课程Add courses to Edit page
讲师可能教授任意数量的课程。Instructors may teach any number of courses. 现在可以通过使用一组复选框来更改课程分配，从而增强讲师编辑页面的性能，如以下屏幕截图所示：Now you'll enhance the Instructor Edit page by adding the ability to change course assignments using a group of check boxes, as shown in the following screen shot:
Course 和 Instructor 实体之间是多对多的关系。The relationship between the Course and Instructor entities is many-to-many. 若要添加和删除关系，可以向 CourseAssignments 联接实体集添加实体和从中删除实体。To add and remove relationships, you add and remove entities to and from the CourseAssignments join entity set.
用于更改讲师所对应的课程的 UI 是一组复选框。The UI that enables you to change which courses an instructor is assigned to is a group of check boxes. 该复选框中会显示数据库中的所有课程，选中讲师当前对应的课程即可。A check box for every course in the database is displayed, and the ones that the instructor is currently assigned to are selected. 用户可以通过选择或清除复选框来更改课程分配。The user can select or clear check boxes to change course assignments. 如果课程的数量过大，建议使用其他方法在视图中呈现数据，但创建或删除关系的方法与操作联接实体的方法相同。If the number of courses were much greater, you would probably want to use a different method of presenting the data in the view, but you'd use the same method of manipulating a join entity to create or delete relationships.
更新讲师控制器Update the Instructors controller
若要为复选框列表的视图提供数据，将使用视图模型类。To provide data to the view for the list of check boxes, you'll use a view model class.
在 SchoolViewModels 文件夹中创建 AssignedCourseData.cs，并将现有代码替换为以下代码 ：Create AssignedCourseData.cs in the SchoolViewModels folder and replace the existing code with the following code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
namespace ContosoUniversity.Models.SchoolViewModels
{
public class AssignedCourseData
{
public int CourseID { get; set; }
public string Title { get; set; }
public bool Assigned { get; set; }
}
}
在 InstructorsController.cs 中，将 HttpGet Edit 方法替换为以下代码。In InstructorsController.cs, replace the HttpGet Edit method with the following code. 突出显示所作更改。The changes are highlighted.
public async Task Edit(int? id)
{
if (id == null)
{
return NotFound();
}
var instructor = await _context.Instructors
.Include(i => i.OfficeAssignment)
.Include(i => i.CourseAssignments).ThenInclude(i => i.Course)
.AsNoTracking()
.FirstOrDefaultAsync(m => m.ID == id);
if (instructor == null)
{
return NotFound();
}
PopulateAssignedCourseData(instructor);
return View(instructor);
}
private void PopulateAssignedCourseData(Instructor instructor)
{
var allCourses = _context.Courses;
var instructorCourses = new HashSet(instructor.CourseAssignments.Select(c => c.CourseID));
var viewModel = new List();
foreach (var course in allCourses)
{
viewModel.Add(new AssignedCourseData
{
CourseID = course.CourseID,
Title = course.Title,
Assigned = instructorCourses.Contains(course.CourseID)
});
}
ViewData["Courses"] = viewModel;
}
该代码为 Courses 导航属性添加了预先加载，并调用新的 PopulateAssignedCourseData 方法使用 AssignedCourseData 视图模型类为复选框数组提供信息。The code adds eager loading for the Courses navigation property and calls the new PopulateAssignedCourseData method to provide information for the check box array using the AssignedCourseData view model class.
PopulateAssignedCourseData 方法中的代码会读取所有 Course 实体，以便使用视图模型类加载课程列表。The code in the PopulateAssignedCourseData method reads through all Course entities in order to load a list of courses using the view model class. 对每门课程而言，该代码都会检查讲师的 Courses 导航属性中是否存在该课程。For each course, the code checks whether the course exists in the instructor's Courses navigation property. 为高效检查某门课程是否被分配给了讲师，可将分配给该讲师的课程放置于 HashSet 集合中。To create efficient lookup when checking whether a course is assigned to the instructor, the courses assigned to the instructor are put into a HashSet collection. 对于讲师分配到的课程，Assigned 属性则设置为 true。The Assigned property is set to true for courses the instructor is assigned to. 视图将使用此属性来确定应将哪些复选框显示为选中状态。The view will use this property to determine which check boxes must be displayed as selected. 最后，该列表会被传递给 ViewData 中的视图。Finally, the list is passed to the view in ViewData.
接下来，添加用户单击“保存”时执行的代码。Next, add the code that's executed when the user clicks Save. 将 EditPost 方法替换为以下代码，并添加一个新方法，用于更新 Instructor 实体的 Courses 导航属性。Replace the EditPost method with the following code, and add a new method that updates the Courses navigation property of the Instructor entity.
[HttpPost]
[ValidateAntiForgeryToken]
public async Task Edit(int? id, string[] selectedCourses)
{
if (id == null)
{
return NotFound();
}
var instructorToUpdate = await _context.Instructors
.Include(i => i.OfficeAssignment)
.Include(i => i.CourseAssignments)
.ThenInclude(i => i.Course)
.FirstOrDefaultAsync(m => m.ID == id);
if (await TryUpdateModelAsync(
instructorToUpdate,
"",
i => i.FirstMidName, i => i.LastName, i => i.HireDate, i => i.OfficeAssignment))
{
if (String.IsNullOrWhiteSpace(instructorToUpdate.OfficeAssignment?.Location))
{
instructorToUpdate.OfficeAssignment = null;
}
UpdateInstructorCourses(selectedCourses, instructorToUpdate);
try
{
await _context.SaveChangesAsync();
}
catch (DbUpdateException /* ex */)
{
//Log the error (uncomment ex variable name and write a log.)
ModelState.AddModelError("", "Unable to save changes. " +
"Try again, and if the problem persists, " +
"see your system administrator.");
}
return RedirectToAction(nameof(Index));
}
UpdateInstructorCourses(selectedCourses, instructorToUpdate);
PopulateAssignedCourseData(instructorToUpdate);
return View(instructorToUpdate);
}private void UpdateInstructorCourses(string[] selectedCourses, Instructor instructorToUpdate)
{
if (selectedCourses == null)
{
instructorToUpdate.CourseAssignments = new List();
return;
}
var selectedCoursesHS = new HashSet(selectedCourses);
var instructorCourses = new HashSet
(instructorToUpdate.CourseAssignments.Select(c => c.Course.CourseID));
foreach (var course in _context.Courses)
{
if (selectedCoursesHS.Contains(course.CourseID.ToString()))
{
if (!instructorCourses.Contains(course.CourseID))
{
instructorToUpdate.CourseAssignments.Add(new CourseAssignment { InstructorID = instructorToUpdate.ID, CourseID = course.CourseID });
}
}
else
{
if (instructorCourses.Contains(course.CourseID))
{
CourseAssignment courseToRemove = instructorToUpdate.CourseAssignments.FirstOrDefault(i => i.CourseID == course.CourseID);
_context.Remove(courseToRemove);
}
}
}
}
现在的方法签名与 HttpGet Edit 方法不同，因此方法名称将从 EditPost 变回 Edit。The method signature is now different from the HttpGet Edit method, so the method name changes from EditPost back to Edit.
视图没有 Course 实体的集合，因此模型绑定器无法自动更新 CourseAssignments 导航属性。Since the view doesn't have a collection of Course entities, the model binder can't automatically update the CourseAssignments navigation property. 可在新的 UpdateInstructorCourses 方法中更新 CourseAssignments 导航属性，而不必使用模型绑定器。Instead of using the model binder to update the CourseAssignments navigation property, you do that in the new UpdateInstructorCourses method. 为此，需要从模型绑定中排除 CourseAssignments 属性。Therefore, you need to exclude the CourseAssignments property from model binding. 此操作无需对调用 TryUpdateModel 的代码进行任何更改，因为使用的是需要显式审批的重载，并且 CourseAssignments 不包括在该列表中。This doesn't require any change to the code that calls TryUpdateModel because you're using the overload that requires explicit approval and CourseAssignments isn't in the include list.
如果未选中任何复选框，则 UpdateInstructorCourses 中的代码将使用空集合初始化 CourseAssignments 导航属性，并返回以下内容：If no check boxes were selected, the code in UpdateInstructorCourses initializes the CourseAssignments navigation property with an empty collection and returns:
private void UpdateInstructorCourses(string[] selectedCourses, Instructor instructorToUpdate)
{
if (selectedCourses == null)
{
instructorToUpdate.CourseAssignments = new List();
return;
}
var selectedCoursesHS = new HashSet(selectedCourses);
var instructorCourses = new HashSet
(instructorToUpdate.CourseAssignments.Select(c => c.Course.CourseID));
foreach (var course in _context.Courses)
{
if (selectedCoursesHS.Contains(course.CourseID.ToString()))
{
if (!instructorCourses.Contains(course.CourseID))
{
instructorToUpdate.CourseAssignments.Add(new CourseAssignment { InstructorID = instructorToUpdate.ID, CourseID = course.CourseID });
}
}
else
{
if (instructorCourses.Contains(course.CourseID))
{
CourseAssignment courseToRemove = instructorToUpdate.CourseAssignments.FirstOrDefault(i => i.CourseID == course.CourseID);
_context.Remove(courseToRemove);
}
}
}
}
之后，代码会循环访问数据库中的所有课程，并逐一检查当前分配给讲师的课程和视图中处于选中状态的课程。The code then loops through all courses in the database and checks each course against the ones currently assigned to the instructor versus the ones that were selected in the view. 为便于高效查找，后两个集合存储在 HashSet 对象中。To facilitate efficient lookups, the latter two collections are stored in HashSet objects.
如果某课程的复选框处于选中状态，但该课程不在 Instructor.CourseAssignments 导航属性中，则会将该课程添加到导航属性中的集合中。If the check box for a course was selected but the course isn't in the Instructor.CourseAssignments navigation property, the course is added to the collection in the navigation property.
private void UpdateInstructorCourses(string[] selectedCourses, Instructor instructorToUpdate)
{
if (selectedCourses == null)
{
instructorToUpdate.CourseAssignments = new List();
return;
}
var selectedCoursesHS = new HashSet(selectedCourses);
var instructorCourses = new HashSet
(instructorToUpdate.CourseAssignments.Select(c => c.Course.CourseID));
foreach (var course in _context.Courses)
{
if (selectedCoursesHS.Contains(course.CourseID.ToString()))
{
if (!instructorCourses.Contains(course.CourseID))
{
instructorToUpdate.CourseAssignments.Add(new CourseAssignment { InstructorID = instructorToUpdate.ID, CourseID = course.CourseID });
}
}
else
{
if (instructorCourses.Contains(course.CourseID))
{
CourseAssignment courseToRemove = instructorToUpdate.CourseAssignments.FirstOrDefault(i => i.CourseID == course.CourseID);
_context.Remove(courseToRemove);
}
}
}
}
如果某课程的复选框未处于选中状态，但该课程存在 Instructor.CourseAssignments 导航属性中，则会从导航属性中删除该课程。If the check box for a course wasn't selected, but the course is in the Instructor.CourseAssignments navigation property, the course is removed from the navigation property.
private void UpdateInstructorCourses(string[] selectedCourses, Instructor instructorToUpdate)
{
if (selectedCourses == null)
{
instructorToUpdate.CourseAssignments = new List();
return;
}
var selectedCoursesHS = new HashSet(selectedCourses);
var instructorCourses = new HashSet
(instructorToUpdate.CourseAssignments.Select(c => c.Course.CourseID));
foreach (var course in _context.Courses)
{
if (selectedCoursesHS.Contains(course.CourseID.ToString()))
{
if (!instructorCourses.Contains(course.CourseID))
{
instructorToUpdate.CourseAssignments.Add(new CourseAssignment { InstructorID = instructorToUpdate.ID, CourseID = course.CourseID });
}
}
else
{
if (instructorCourses.Contains(course.CourseID))
{
CourseAssignment courseToRemove = instructorToUpdate.CourseAssignments.FirstOrDefault(i => i.CourseID == course.CourseID);
_context.Remove(courseToRemove);
}
}
}
}
更新讲师视图Update the Instructor views
在 Views/Instructors/Edit.cshtml 中，通过在“办公室”字段的 div 元素之后和“保存”按钮的 div 元素之前添加以下代码，以便添加带有一系列复选框的“课程”字段 。In Views/Instructors/Edit.cshtml, add a Courses field with an array of check boxes by adding the following code immediately after the div elements for the Office field and before the div element for the Save button.
备注
将代码粘贴到 Visual Studio 中时，换行符会发生更改，从而导致代码中断。When you paste the code in Visual Studio, line breaks might be changed in a way that breaks the code. 如果粘贴后代码看起来不同，按 Ctrl+Z 一次可撤消自动格式设置。If the code looks different after pasting, press Ctrl+Z one time to undo the automatic formatting. 这样可以修复换行符，使其看起来如此处所示。This will fix the line breaks so that they look like what you see here. 缩进不一定要完美，但 @:
、@:、@: 和 @: 行必须各成一行(如下所示)，否则会出现运行时错误。The indentation doesn't have to be perfect, but the @:, @:, @:, and @: lines must each be on a single line as shown or you'll get a runtime error. 选中新的代码块后，按 Tab 三次，使新代码与现有代码对齐。With the block of new code selected, press Tab three times to line up the new code with the existing code. Visual Studio 2019 修复了此问题。This problem is fixed in Visual Studio 2019.@{
int cnt = 0;
List courses = ViewBag.Courses;
foreach (var course in courses)
{
if (cnt++ % 3 == 0)
{
@:
}
@:
name="selectedCourses"
value="@course.CourseID"
@(Html.Raw(course.Assigned ? "checked=\"checked\"" : "")) />
@course.CourseID @: @course.Title
@:
}
@:
}
此代码将创建一个具有三列的 HTML 表。This code creates an HTML table that has three columns. 每个列中都有一个复选框，随后是一段由课程编号和标题组成的描述文字。In each column is a check box followed by a caption that consists of the course number and title. 所有复选框都具有相同的名称，即 selectedCourses，以告知模型绑定器将它们视为一组。The check boxes all have the same name ("selectedCourses"), which informs the model binder that they're to be treated as a group. 每个复选框的值特性被设置为 CourseID 的值。The value attribute of each check box is set to the value of CourseID. 发布页面时，模型绑定器会向控制器传递一个数组，其中只包括所选复选框的 CourseID 值。When the page is posted, the model binder passes an array to the controller that consists of the CourseID values for only the check boxes which are selected.
这些复选框最开始呈现时，对于分配给讲师的课程的复选框，其特性处于选中状态。When the check boxes are initially rendered, those that are for courses assigned to the instructor have checked attributes, which selects them (displays them checked).
运行应用，选择“讲师”选项卡，然后单击讲师页面上的“编辑”以查看“编辑”页面 。Run the app, select the Instructors tab, and click Edit on an instructor to see the Edit page.
更改某些课程分配并单击“保存”。Change some course assignments and click Save. 所作更改将反映在索引页上。The changes you make are reflected on the Index page.
备注
此处所使用的编辑讲师课程数据的方法适用于数量有限的课程。The approach taken here to edit instructor course data works well when there's a limited number of courses. 若是远大于此的集合，则需要使用不同的 UI 和不同的更新方法。For collections that are much larger, a different UI and a different updating method would be required.
更新“删除”页Update Delete page
在 InstructorsController.cs 中，删除 DeleteConfirmed 方法，并在其位置插入以下代码。In InstructorsController.cs, delete the DeleteConfirmed method and insert the following code in its place.
[HttpPost, ActionName("Delete")]
[ValidateAntiForgeryToken]
public async Task DeleteConfirmed(int id)
{
Instructor instructor = await _context.Instructors
.Include(i => i.CourseAssignments)
.SingleAsync(i => i.ID == id);
var departments = await _context.Departments
.Where(d => d.InstructorID == id)
.ToListAsync();
departments.ForEach(d => d.InstructorID = null);
_context.Instructors.Remove(instructor);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
此代码会更改以下内容：This code makes the following changes:
对 CourseAssignments 导航属性执行预先加载。Does eager loading for the CourseAssignments navigation property. 必须包括此内容，否则 EF 不知道相关的 CourseAssignment 实体，也不会删除它们。You have to include this or EF won't know about related CourseAssignment entities and won't delete them. 为避免在此处阅读它们，可以在数据库中配置级联删除。To avoid needing to read them here you could configure cascade delete in the database.
如果要删除的讲师被指派为任何系的管理员，则需从这些系中删除该讲师分配。If the instructor to be deleted is assigned as administrator of any departments, removes the instructor assignment from those departments.
向“创建”页添加办公室位置和课程Add office location and courses to Create page
在 InstructorsController.cs 中，删除 HttpGet 和 HttpPost Create 方法，然后在其位置添加以下代码：In InstructorsController.cs, delete the HttpGet and HttpPost Create methods, and then add the following code in their place:
public IActionResult Create()
{
var instructor = new Instructor();
instructor.CourseAssignments = new List();
PopulateAssignedCourseData(instructor);
return View();
}
// POST: Instructors/Create
[HttpPost]
[ValidateAntiForgeryToken]
public async Task Create([Bind("FirstMidName,HireDate,LastName,OfficeAssignment")] Instructor instructor, string[] selectedCourses)
{
if (selectedCourses != null)
{
instructor.CourseAssignments = new List();
foreach (var course in selectedCourses)
{
var courseToAdd = new CourseAssignment { InstructorID = instructor.ID, CourseID = int.Parse(course) };
instructor.CourseAssignments.Add(courseToAdd);
}
}
if (ModelState.IsValid)
{
_context.Add(instructor);
await _context.SaveChangesAsync();
return RedirectToAction(nameof(Index));
}
PopulateAssignedCourseData(instructor);
return View(instructor);
}
此代码与 Edit 方法中所示内容类似，只是最开始未选择任何课程。This code is similar to what you saw for the Edit methods except that initially no courses are selected. HttpGet Create 方法调用 PopulateAssignedCourseData 方法不是因为可能有课程处于选中状态，而是为了在视图中为 foreach 循环提供空集合(否则该视图代码将引发空引用异常)。The HttpGet Create method calls the PopulateAssignedCourseData method not because there might be courses selected but in order to provide an empty collection for the foreach loop in the view (otherwise the view code would throw a null reference exception).
检查是否存在验证错误并向数据库添加新讲师之前，HttpPost Create 方法会将每个选定课程添加到 CourseAssignments 导航属性。The HttpPost Create method adds each selected course to the CourseAssignments navigation property before it checks for validation errors and adds the new instructor to the database. 即使存在模型错误也会添加课程，因此出现模型错误(例如用户键入了无效的日期)并且页面重新显示并出现错误消息时，所作的任何课程选择都会自动还原。Courses are added even if there are model errors so that when there are model errors (for an example, the user keyed an invalid date), and the page is redisplayed with an error message, any course selections that were made are automatically restored.
请注意，为了能够向 CourseAssignments 导航属性添加课程，必须将该属性初始化为空集合：Notice that in order to be able to add courses to the CourseAssignments navigation property you have to initialize the property as an empty collection:
instructor.CourseAssignments = new List();
除了在控制器代码中进行此操作之外，还可以在“导师”模型中进行此操作，方法是将该属性更改为不存在集合时自动创建集合，如以下示例所示：As an alternative to doing this in controller code, you could do it in the Instructor model by changing the property getter to automatically create the collection if it doesn't exist, as shown in the following example:
private ICollection _courseAssignments;
public ICollection CourseAssignments
{
get
{
return _courseAssignments ?? (_courseAssignments = new List());
}
set
{
_courseAssignments = value;
}
}
如果通过这种方式修改 CourseAssignments 属性，则可以删除控制器中的显式属性初始化代码。If you modify the CourseAssignments property in this way, you can remove the explicit property initialization code in the controller.
在 Views/Instructor/Create.cshtml 中，添加一个办公室位置文本框和课程的复选框，然后按“提交”按钮。In Views/Instructor/Create.cshtml, add an office location text box and check boxes for courses before the Submit button.
@{
int cnt = 0;
List courses = ViewBag.Courses;
foreach (var course in courses)
{
if (cnt++ % 3 == 0)
{
@:
}
@:
name="selectedCourses"
value="@course.CourseID"
@(Html.Raw(course.Assigned ? "checked=\"checked\"" : "")) />
@course.CourseID @: @course.Title
@:
}
@:
}
通过运行应用并创建讲师来进行测试。Test by running the app and creating an instructor.
处理事务Handling Transactions
如 CRUD 教程中所述，Entity Framework 隐式实现事务。As explained in the CRUD tutorial, the Entity Framework implicitly implements transactions. 如果需要更多控制操作(例如，如果想要在事务中包含在 Entity Framework 外部完成的操作)，请参阅事务。For scenarios where you need more control -- for example, if you want to include operations done outside of Entity Framework in a transaction -- see Transactions.
获取代码Get the code
后续步骤Next steps
在本教程中，你将了解：In this tutorial, you:
已自定义“课程”页Customized Courses pages
已添加“讲师编辑”页Added Instructors Edit page
已向“编辑”页添加课程Added courses to Edit page
已更新“删除”页Updated Delete page
已向“创建”页添加办公室位置和课程Added office location and courses to Create page
请继续阅读下一篇教程，了解如何处理并发冲突。Advance to the next tutorial to learn how to handle concurrency conflicts.