索引：

目錄索引

Adding a New Field

添加一個新字段

2016-10-14?3 分鐘閱讀時長?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.

在本節,我們將用EF的Code First 增加一個新字段并變更到數據庫中.

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.

當你使用 EF Code First 自動的創建一個數據庫，Code First將會向數據庫增加一張表，他會自動追蹤數據庫結構的變化并同步結構的變化。

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

如果他們未同步，EF會拋出異常。這使得代碼與DB保持一致變得簡單。

Adding a Rating Property to the Movie Model

給 Movie 模型添加一個等級字段

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

打開 Models/Movie.cs 文件，并添加 Rating 屬性字段：

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).

編譯應用。

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.

因為你在 Movie 類中新增了一個字段，你需要更新綁定，這樣這個新字段才能被包含。

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 屬性字段：

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.

你同樣需要更新視圖模板，以便在瀏覽器上顯示，新建，編輯 Rating 字段。

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

編輯 /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 復制、粘貼 前邊的 "form group" ，并讓智能提示幫助你完成字段的更新。

IntelliSense works with?Tag Helpers.

智能提示使用 Tag Helpers 來完成工作。

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

筆記：現在已是 15.3.1 版本了，此句不翻譯~

This will be fixed in the next release.

此句不翻譯~

?

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:

在將 新字段包含到 DB之前 程序時不會正確運行的，他會拋出一個 SqlException 異常：

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.)

你將會看到錯誤，因為更新后的model不同于DB表的結構。

There are a few approaches to resolving the error:

下面是一些解決問題的方法：

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

讓EF自動刪除并基于 模型類 結構重建DB結構。

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

在早期的開發周期中，當你在一個測試庫上開發時這是一個非常方便的做法；

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

他允許你讓model與db結構一起快速迭代進化。

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!

這么做的缺點是會丟失現存庫中的所有數據，在生產上我們是不希望使用這種方法的！

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

使用初始化器來自動種植一些DB數據，是一種常使用的提高生產力的開發做法。

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

明確的更新已存在數據庫的結構，讓它匹配代碼中的模型類。

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

這種做法的優點是可以讓你保持數據庫中已存在的數據。

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

你可以人工的或使用腳本自動的來變更DB。

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

使用Code First遷移來更新數據庫結構。

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

在本教程中，我們會使用 Code First 遷移的做法。

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

更新 SeedData 類，讓它為新字段提供值。

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

如下是一個變更示例，你需要為每一個 new Movie 加上變化。

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.

編譯解決方案。

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

在 Tools 菜單，選擇 NuGet Package Manager > Package Manager Console 子菜單：

?

In the PMC, enter the following commands:

在 PMC 中，鍵入以下命令：

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 命令告訴 遷移框架 檢查當前的 Movie 類并與DB結構做比較并創建出合適的代碼更新數據庫使其匹配新的model類。

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

"Rating" 是任意命名的，它被用來命名遷移文件名。

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

使用有意義的名字命名遷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.

如果你刪除了DB中的數據記錄，初始化類將重新種植DB并包含 Rating 字段。

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

你可以在瀏覽器上的刪除鏈接或在 SSOX 管理器界面上這么做。

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

運行程序并檢查你可以對movie增刪改查新的 Rating 字段。

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

你同樣應該在 Edit,?Details, and?Delete 視圖模板上增加 Rating 字段。

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　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　2017-08-22 11:22 周二

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轉載于:https://www.cnblogs.com/Meng-NET/p/7410756.html

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