012.Adding a New Field --【添加一个新字段】

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目录索引

Adding a New Field

添加一个新字段

2016-10-14 3 分钟阅读时长作者

By Rick Anderson

In this section you'll use Entity Framework Code First Migrations to add a new field to the model and migrate that change to the database.

在本节,我们将用EF的Code First 增加一个新字段并变更到数据库中.

When you use EF Code First to automatically create a database, Code First adds a table to the database to help track whether the schema of the database is in sync with the model classes it was generated from.

当你使用 EF Code First 自动的创建一个数据库，Code First将会向数据库增加一张表，他会自动追踪数据库结构的变化并同步结构的变化。

If they aren't in sync, EF throws an exception. This makes it easier to find inconsistent database/code issues.

如果他们未同步，EF会抛出异常。这使得代码与DB保持一致变得简单。

Adding a Rating Property to the Movie Model

给 Movie 模型添加一个等级字段

Open the Models/Movie.cs file and add a Rating property:

打开 Models/Movie.cs 文件，并添加 Rating 属性字段：

 1 public class Movie
 2 
 3 {
 4 
 5     public int ID { get; set; }
 6 
 7     public string Title { get; set; }
 8 
 9  
10 
11     [Display(Name = "Release Date")]
12 
13     [DataType(DataType.Date)]
14 
15     public DateTime ReleaseDate { get; set; }
16 
17     public string Genre { get; set; }
18 
19     public decimal Price { get; set; }
20 
21     public string Rating { get; set; }
22 
23 }

C# code

Build the app (Ctrl+Shift+B).

编译应用。

Because you've added a new field to the Movie class, you also need to update the binding white list so this new property will be included.

因为你在 Movie 类中新增了一个字段，你需要更新绑定，这样这个新字段才能被包含。

In MoviesController.cs, update the [Bind] attribute for both the Create and Edit action methods to include the Rating property:

在 MoviesController.cs 文件中，更新 Create 、 Edit 方法的 [Bind] ，以包含 Rating 属性字段：

1 [Bind("ID,Title,ReleaseDate,Genre,Price,Rating")]

C# Code

You also need to update the view templates in order to display, create and edit the new Rating property in the browser view.

你同样需要更新视图模板，以便在浏览器上显示，新建，编辑 Rating 字段。

Edit the /Views/Movies/Index.cshtml file and add a Rating field:

编辑 /Views/Movies/Index.cshtml 文件并增加 Rating 字段：

 1 <table class="table">
 2 
 3     <thead>
 4 
 5         <tr>
 6 
 7             <th>
 8 
 9                 @Html.DisplayNameFor(model => model.movies[0].Title)
10 
11             </th>
12 
13             <th>
14 
15                 @Html.DisplayNameFor(model => model.movies[0].ReleaseDate)
16 
17             </th>
18 
19             <th>
20 
21                 @Html.DisplayNameFor(model => model.movies[0].Genre)
22 
23             </th>
24 
25             <th>
26 
27                 @Html.DisplayNameFor(model => model.movies[0].Price)
28 
29             </th>
30 
31             <th>
32 
33                 @Html.DisplayNameFor(model => model.movies[0].Rating)
34 
35             </th>
36 
37             <th></th>
38 
39         </tr>
40 
41     </thead>
42 
43     <tbody>
44 
45         @foreach (var item in Model.movies)
46 
47         {
48 
49             <tr>
50 
51                 <td>
52 
53                     @Html.DisplayFor(modelItem => item.Title)
54 
55                 </td>
56 
57                 <td>
58 
59                     @Html.DisplayFor(modelItem => item.ReleaseDate)
60 
61                 </td>
62 
63                 <td>
64 
65                     @Html.DisplayFor(modelItem => item.Genre)
66 
67                 </td>
68 
69                 <td>
70 
71                     @Html.DisplayFor(modelItem => item.Price)
72 
73                 </td>
74 
75                 <td>
76 
77                     @Html.DisplayFor(modelItem => item.Rating)
78 
79                 </td>
80 
81                 <td>

HTML Code

Update the /Views/Movies/Create.cshtml with a Rating field.

更新 /Views/Movies/Create.cshtml 文件，增加 Rating 字段。

You can copy/paste the previous "form group" and let intelliSense help you update the fields.

你可以复制、粘贴前边的 "form group" ，并让智能提示帮助你完成字段的更新。

IntelliSense works with Tag Helpers.

智能提示使用 Tag Helpers 来完成工作。

Note: In the RTM verison of Visual Studio 2017 you need to install the Razor Language Services for Razor intelliSense.

笔记：现在已是 15.3.1 版本了，此句不翻译~

This will be fixed in the next release.

此句不翻译~

The app won't work until we update the DB to include the new field. If you run it now, you'll get the following SqlException:

在将新字段包含到 DB之前程序时不会正确运行的，他会抛出一个 SqlException 异常：

SqlException: Invalid column name 'Rating'.

TXT Code

You're seeing this error because the updated Movie model class is different than the schema of the Movie table of the existing database. (There's no Rating column in the database table.)

你将会看到错误，因为更新后的model不同于DB表的结构。

There are a few approaches to resolving the error:

下面是一些解决问题的方法：

1.Have the Entity Framework automatically drop and re-create the database based on the new model class schema.

让EF自动删除并基于模型类结构重建DB结构。

This approach is very convenient early in the development cycle when you are doing active development on a test database;

在早期的开发周期中，当你在一个测试库上开发时这是一个非常方便的做法；

it allows you to quickly evolve the model and database schema together.

他允许你让model与db结构一起快速迭代进化。

The downside, though, is that you lose existing data in the database — so you don't want to use this approach on a production database!

这么做的缺点是会丢失现存库中的所有数据，在生产上我们是不希望使用这种方法的！

Using an initializer to automatically seed a database with test data is often a productive way to develop an application.

使用初始化器来自动种植一些DB数据，是一种常使用的提高生产力的开发做法。

2.Explicitly modify the schema of the existing database so that it matches the model classes.

明确的更新已存在数据库的结构，让它匹配代码中的模型类。

The advantage of this approach is that you keep your data.

这种做法的优点是可以让你保持数据库中已存在的数据。

You can make this change either manually or by creating a database change script.

你可以人工的或使用脚本自动的来变更DB。

3.Use Code First Migrations to update the database schema.

使用Code First迁移来更新数据库结构。

For this tutorial, we'll use Code First Migrations.

在本教程中，我们会使用 Code First 迁移的做法。

Update the SeedData class so that it provides a value for the new column.

更新 SeedData 类，让它为新字段提供值。

A sample change is shown below, but you'll want to make this change for each new Movie.

如下是一个变更示例，你需要为每一个 new Movie 加上变化。

 1 new Movie
 2 
 3 {
 4 
 5     Title = "When Harry Met Sally",
 6 
 7     ReleaseDate = DateTime.Parse("1989-1-11"),
 8 
 9     Genre = "Romantic Comedy",
10 
11     Rating = "R",
12 
13     Price = 7.99M
14 
15 },

C# Code

Build the solution.

编译解决方案。

From the Tools menu, select NuGet Package Manager > Package Manager Console.

在 Tools 菜单，选择 NuGet Package Manager > Package Manager Console 子菜单：

In the PMC, enter the following commands:

在 PMC 中，键入以下命令：

1 Add-Migration Rating
2 
3 Update-Database

Bash Code

The Add-Migration command tells the migration framework to examine the current Movie model with the current Movie DB schema and create the necessary code to migrate the DB to the new model.

Add-Migration 命令告诉迁移框架检查当前的 Movie 类并与DB结构做比较并创建出合适的代码更新数据库使其匹配新的model类。

The name "Rating" is arbitrary and is used to name the migration file.

"Rating" 是任意命名的，它被用来命名迁移文件名。

It's helpful to use a meaningful name for the migration file.

使用有意义的名字命名迁移文件是非常有益的。

If you delete all the records in the DB, the initialize will seed the DB and include the Rating field.

如果你删除了DB中的数据记录，初始化类将重新种植DB并包含 Rating 字段。

You can do this with the delete links in the browser or from SSOX.

你可以在浏览器上的删除链接或在 SSOX 管理器界面上这么做。

Run the app and verify you can create/edit/display movies with a Rating field.

运行程序并检查你可以对movie增删改查新的 Rating 字段。

You should also add the Rating field to the Edit, Details, and Delete view templates.