Code First Migrations更新数据库结构（数据迁移）
背景 code first起初当修改model后，要持久化至数据库中时，总要把原数据库给删除掉再创建（DropCreateDatabaseIfModelChanges），此时就会产生一个问题，当我们的旧数据库中包含一些测试数据时，当持久化更新后，原数据将全部丢失，故我们可以引入EF的数据
背景
code first起初当修改model后，要持久化至数据库中时，总要把原数据库给删除掉再创建（DropCreateDatabaseIfModelChanges），此时就会产生一个问题，当我们的旧数据库中包含一些测试数据时，当持久化更新后，原数据将全部丢失，故我们可以引入EF的数据迁移功能来完成。
要求
- 已安装NuGet
过程示例
//原model
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
public class Lesson {
public int lessonID { get; set; }
[Required]
[MaxLength(50)]
public string lessonName { get; set; }
[Required]
public string teacherName { get; set; }
public virtual UserInfo UserInfo{get;set;}
}//新model
using System.Collections;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
public class Lesson {
public int lessonID { get; set; }
[Required]
[MaxLength(50)]
public string lessonName { get; set; }
[Required]
[MaxLength(10)]
public string teacherName { get; set; }
public virtual UserInfo UserInfo{get;set;}
}
接下来，我们将开始持久化此model至数据库中（我们现在只是对属性作修改，此时数据库中此字段的长度为nvarchar(max)，并不是nvarchar(10)）
1：在config中配置数据库连接：
<connectionstrings>
<add name="TestUsersDB" connectionstring="Integrated Security=SSPI;Persist Security Info=False;Initial Catalog=TestUsersDB;Data Source=XCL-PC\SQLEXPRESS" providername="System.Data.SqlClient"></add>
</connectionstrings>2：打开NuGet控制台：
3：运行命令Enable-Migrations
可能会出现如下错误：
Checking if the context targets an existing database...
Detected database created with a database initializer. Scaffolded migration '201212090821166_InitialCreate' corresponding to existing database. To use an automatic migration instead, delete the Migrations folder and re-run Enable-Migrations specifying the -EnableAutomaticMigrations
parameter.
Code First Migrations enabled for project MvcApplication1.
此时项目会出现如下文件夹：
打开configuation.cs，将作出如下修改：
public Configuration()
{
AutomaticMigrationsEnabled = true;
}再次执行Update-Database：
因为我把长度从max改为10，在更新数据结构时，它认为此操作会导致数据丢失，如下：
Specify the '-Verbose' flag to view the SQL statements being applied to the target database.
No pending code-based migrations.
Applying automatic migration: 201212090848057_AutomaticMigration.
Automatic migration was not applied because it would result in data loss.
如果确保没事，只需给此命令加个强制执行的参数即可：
Enable-Migrations -Force
最后再次执行：Update-Database
数据库中的原数据也没有丢失！
3：
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