asp.net MVC5--為數(shù)據(jù)庫(kù)新增字段（涉及數(shù)據(jù)庫(kù)遷移技術(shù)）

Setting up Code First Migrations for Model Changes--為模型更改做數(shù)據(jù)庫(kù)遷移。

1.打開(kāi)資源管理器，在App_Data文件夾下，找到movies.mdf數(shù)據(jù)庫(kù)文件，如果沒(méi)有看到點(diǎn)擊顯示所有文件。

2.刪掉movies.mdf數(shù)據(jù)庫(kù)文件，并編譯項(xiàng)目。確保沒(méi)有報(bào)錯(cuò)。

3.找到工具菜單欄下面的NuGet程序包管理器---程序包管理器控制臺(tái)，如圖所示：

4，在程序包管理器控制臺(tái)中，輸入：Enable-Migrations -ContextTypeName MvcMovie.Models.MovieDBContext

（注意：MvcMovie.Models.MovieDBContext 項(xiàng)目名.Models.項(xiàng)目數(shù)據(jù)上下文）

按enter鍵之后，可以看到：

5，數(shù)據(jù)庫(kù)遷移之后，VS中自動(dòng)為我們生成了一個(gè)Migrations文件夾，里面有一個(gè)Configuration.cs文件

6.打開(kāi)Configuration.cs文件，引用命名空間：

using MvcMovie.Models;

然后在Seed方法中寫(xiě)上：

 1 PRotected override void Seed(MvcMovie.Models.MovieDBContext context) 2 { 3     context.Movies.AddOrUpdate( i => i.Title, 4         new Movie 5         { 6             Title = "When Harry Met Sally", 7             ReleaseDate = DateTime.Parse("1989-1-11"), 8             Genre = "Romantic Comedy", 9             Price = 7.99M10         },11 12          new Movie13          {14              Title = "Ghostbusters ",15              ReleaseDate = DateTime.Parse("1984-3-13"),16              Genre = "Comedy",17              Price = 8.99M18          },19 20          new Movie21          {22              Title = "Ghostbusters 2",23              ReleaseDate = DateTime.Parse("1986-2-23"),24              Genre = "Comedy",25              Price = 9.99M26          },27 28        new Movie29        {30            Title = "Rio Bravo",31            ReleaseDate = DateTime.Parse("1959-4-15"),32            Genre = "Western",33            Price = 3.99M34        }35    );36    37 }

7.Code First Migrations calls theSeedmethod after every migration (that is, callingupdate-databasein the Package Manager Console), and this method updates rows that have already been inserted, or inserts them if they don't exist yet.

這句話的意思是：在每一次數(shù)據(jù)庫(kù)遷移的時(shí)候，這個(gè)seed方法都會(huì)被調(diào)用，這個(gè)方法更新已經(jīng)插入的行數(shù)據(jù)，或者插入行，如果這個(gè)行數(shù)據(jù)不存在。

8.下面的方法起到了一個(gè)更新插入的作用：

1 context.Movies.AddOrUpdate(i => i.Title,2     new Movie3     {4         Title = "When Harry Met Sally",5         ReleaseDate = DateTime.Parse("1989-1-11"),6         Genre = "Romantic Comedy",7         Rating = "PG",8         Price = 7.99M9     }

9.*因?yàn)镾eed方法，在每次數(shù)據(jù)庫(kù)遷移的時(shí)候，都會(huì)執(zhí)行。你不能僅僅是插入數(shù)據(jù)，因?yàn)槟銓⒁迦氲臄?shù)據(jù)，將在第一次數(shù)據(jù)庫(kù)遷移結(jié)束之后，已經(jīng)存在數(shù)據(jù)庫(kù)中；

*更新插入的操作可以預(yù)防錯(cuò)誤，通過(guò)阻止你，插入已經(jīng)存在的數(shù)據(jù)到數(shù)據(jù)庫(kù)中。但是它有個(gè)缺點(diǎn)：它重載了，所有你在測(cè)試項(xiàng)目時(shí)候改變的數(shù)據(jù)；

因?yàn)橛行y(cè)試數(shù)據(jù)，你不想改變：比如，你測(cè)試的時(shí)候，改變了數(shù)據(jù)，但是你不想這個(gè)數(shù)據(jù)在數(shù)據(jù)庫(kù)更新的時(shí)候，發(fā)生改變。這個(gè)時(shí)候你可以做一個(gè)新增的操作：新增一個(gè)數(shù)據(jù)庫(kù)中不存在的數(shù)據(jù)。

10.看一下這句代碼吧：context.Movies.AddOrUpdate(i => i.Title,這第一個(gè)傳到AddOrUpdate方法的參數(shù)，指定了一個(gè)屬性，用來(lái)檢查是否已經(jīng)存在相同的行數(shù)據(jù)，對(duì)于我這個(gè)項(xiàng)目來(lái)說(shuō)，我這個(gè)Title，可以做為這個(gè)屬性，因?yàn)樗贚ist中每次都是唯一的；This code assumes that titiles are unique. If you manually add a duplicate title, you'll get the following exception the next time you perform a migration.

Sequence contains more than one element 我們假想Title是唯一的，如果你手動(dòng)添加了重復(fù)的Title，你將會(huì)在下次數(shù)據(jù)庫(kù)遷移的時(shí)候，報(bào)一個(gè)錯(cuò)誤，了解更多，請(qǐng)參考鏈接的文章，哈哈，純?nèi)斯?a href="http://www.survivalescaperooms.com/tools/zaixianfanyi.asp">翻譯的哦。因?yàn)椴┲飨矚g英語(yǔ)，所以還是看外國(guó)人的資料學(xué)習(xí)編程了。MSDN很不錯(cuò)的，For more information about theAddOrUpdatemethod, seeTake care with EF 4.3 AddOrUpdate Method..11.現(xiàn)在我們來(lái)編譯一下整個(gè)項(xiàng)目吧，如果這這里不編譯的話，后面的步驟中將會(huì)出錯(cuò)誤。12.The next step is to create aDbMigrationclass for the initial migration. This migration creates a new database, that's why you deleted themovie.mdffile in a previous step.這句話的意思是：我們接下來(lái)要為初始化數(shù)據(jù)庫(kù)遷移，創(chuàng)建一個(gè)DBMigration類(lèi)，這個(gè)數(shù)據(jù)庫(kù)遷移創(chuàng)建一個(gè)新的數(shù)據(jù)庫(kù)，這也就是我們前面刪掉Movie.mdf文件的原因。13.在程序包管理器控制臺(tái)中輸入：add-migration Initial我們看到：

14.Code First Migrations creates another class file in theMigrationsfolder (with the name{DateStamp}_Initial.cs), and this class contains code that creates the database schema. The migration filename is pre-fixed with a timestamp to help with ordering. Examine the{DateStamp}_Initial.csfile, it contains the instructions to create theMoviestable for the Movie DB. When you update the database in the instructions below, this{DateStamp}_Initial.csfile will run and create the the DB schema. Then theSeedmethod will run to populate the DB with test data.

這段話的意思是：Code First遷移，在Migration文件下，創(chuàng)建了另外一個(gè)類(lèi)（類(lèi)的文件名稱是：時(shí)間_Initiaal.cs），并且這個(gè)類(lèi)包含了創(chuàng)建數(shù)據(jù)庫(kù)的代碼。這個(gè)文件以時(shí)間的命名方式便于排序管理。檢查這個(gè)文件，它包含了怎么為MovieDB創(chuàng)建Moviess數(shù)據(jù)庫(kù)表。當(dāng)你按照下面的指令（等會(huì)我在控制器管理控制臺(tái)中輸入的指定），更新數(shù)據(jù)庫(kù)的時(shí)候，這個(gè)文件會(huì)執(zhí)行，并且創(chuàng)建數(shù)據(jù)庫(kù)，然后這個(gè)Seed方法，也將會(huì)執(zhí)行，為數(shù)據(jù)庫(kù)生成測(cè)試數(shù)據(jù)。

先看看看這個(gè)文件里面的代碼是啥樣的吧：

 1 namespace MvcMovie.Migrations 2 { 3     using System; 4     using System.Data.Entity.Migrations; 5      6     public partial class Initial : DbMigration 7     { 8         public override void Up() 9         {10             CreateTable(11                 "dbo.Movies",12                 c => new13                     {14                         ID = c.Int(nullable: false, identity: true),15                         Title = c.String(),16                         ReleaseDate = c.DateTime(nullable: false),17                         Genre = c.String(),18                         Price = c.Decimal(nullable: false, precision: 18, scale: 2),19                     })20                 .PrimaryKey(t => t.ID);21             22         }23         24         public override void Down()25         {26             DropTable("dbo.Movies");27         }28     }29 }

同樣看看我們之前的Migration里面Configuration.cs代碼吧：

 1 namespace MvcMovie.Migrations 2 { 3     using MvcMovie.Models; 4     using System; 5     using System.Data.Entity; 6     using System.Data.Entity.Migrations; 7     using System.Linq; 8  9     internal sealed class Configuration : DbMigrationsConfiguration<MvcMovie.Models.MovieDBContext>10     {11         public Configuration()12         {13             AutomaticMigrationsEnabled = false;14         }15 16         protected override void Seed(MvcMovie.Models.MovieDBContext context)17         {18             context.Movies.AddOrUpdate(i => i.Title,19         new Movie20         {21             Title = "When Harry Met Sally",22             ReleaseDate = DateTime.Parse("1989-1-11"),23             Genre = "Romantic Comedy",24             Price = 7.99M25         },26 27          new Movie28          {29              Title = "Ghostbusters ",30              ReleaseDate = DateTime.Parse("1984-3-13"),31              Genre = "Comedy",32              Price = 8.99M33          },34 35          new Movie36          {37              Title = "Ghostbusters 2",38              ReleaseDate = DateTime.Parse("1986-2-23"),39              Genre = "Comedy",40              Price = 9.99M41          },42 43        new Movie44        {45            Title = "Rio Bravo",46            ReleaseDate = DateTime.Parse("1959-4-15"),47            Genre = "Western",48            Price = 3.99M49        }50    );51         }52     }53 }

現(xiàn)在我們?cè)冢绦虬芾砥骺刂婆_(tái)中輸入這個(gè)指令來(lái)創(chuàng)建數(shù)據(jù)庫(kù)，并運(yùn)行seed方法：

update-database

我們可以看到：

If you get an error that indicates a table already exists and can't be created, it is probably because you ran the application after you deleted the database and before you executedupdate-database. In that case, delete theMovies.mdffile again and retry theupdate-databasecommand. If you still get an error, delete the migrations folder and contents