Entity Framework 6 Code First 多个Dbcontext的迁移方法

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Entity Framework code first migrations（迁移）允许我们基于实体来创建和更新数据库. Entity Framework5 code first migrations只能更新一个DbContext. Now, Entity Framework6 code first migrations 现在可以针对每个数据库实例更新多个DbContext。下面我们看看在EF6中如何实现该功能.

原文链接：http://www.dotnet-tricks.com/Tutorial/entityframework/2VOa140214-Entity-Framework-6-Code-First-Migrations-with-Multiple-Data-Contexts.html

假设你有两个 DbContext， DataContext和UserDataContext. 你需要合并这两个DataContext到一个数据库实例中.

实体模型

public class User
{
 public int UserID { set; get; }
 public string FirstName { set; get; }
 public string LastName { set; get; }
}
 
public class Role
{
 public int RoleID { set; get; }
 public string RolesName { set; get; }
} 
 
public class Order
{
 public int OrderID { set; get; }
 public int Quantity { set; get; }
 public double Amount { set; get; }
 public DateTime Date { set; get; }
}

数据库上下文

//first DbContext
namespace MultiDataContextMigrations.Models
{
public class DataContext : DbContext
{
 public DataContext()
 : base("DefaultConnection")
 {
 
 }
 
 protected override void OnModelCreating(DbModelBuilder modelBuilder)
 {
 //TODO:Define mapping
 }
 
 public DbSet Users { get; set; }
 public DbSet Orders { get; set; }
}
}
//second DbContext
namespace MultiDataContextMigrations.Models
{
public class UserDataContext : DbContext
{
 public UserDataContext():base("DefaultConnection")
 {
 }
 
 protected override void OnModelCreating(DbModelBuilder modelBuilder)
 {
 //TODO:Define mapping
 }
 
 public DbSet Users { get; set; }
 public DbSet Roles { get; set; }
}
}

实例1 : 一个项目中含有多个 DbContext

假设你在一个项目中含有两个DbContext，如下图：

在vs2012和vs2013的程序包管理控制台中，运行下面的命令可以分别将同一项目中的多个Dbcontex进行迁移:

enable-migrations -ContextTypeName <DbContext-Name-with-Namespaces> -MigrationsDirectory:<Migrations-Directory-Name>
Add-Migration -configuration <DbContext-Migrations-Configuration-Class-with-Namespaces> <Migrations-Name>
Update-Database -configuration <DbContext-Migrations-Configuration-Class-with-Namespaces> -Verbose

迁移第一个 DbContext(DataContext)

enable-migrations -ContextTypeName MultiDataContextMigrations.Models.DataContext -MigrationsDirectory:DataContextMigrations
Add-Migration -configuration MultiDataContextMigrations.DataContextMigrations.Configuration Initial
Update-Database -configuration MultiDataContextMigrations.DataContextMigrations.Configuration -Verbose

如果你在命令行执行了上面的命令，我们会看到如下的实际效果:

首先，一个迁移文件夹被创建，文件夹中有一个config类文件.

internal sealed class Configuration : DbMigrationsConfiguration
{
 public Configuration()
 {
 AutomaticMigrationsEnabled = false;
 //Helps to migrate this DbContext to database
 MigrationsDirectory = @"DataContextMigrations"; 
 }
 protected override void Seed(MultiDataContextMigrations.Models.DataContext context)
 {
 // This method will be called after migrating to the latest version.
 }
}

第二，对应我们添加的迁移项，会生成一个类文件，显示更新的内容，本例中为创建表：

public partial class Initial : DbMigration
{
 public override void Up()
 {
 CreateTable(
 "dbo.Orders",
 c => new
 {
 OrderID = c.Int(nullable: false, identity: true),
 Quantity = c.Int(nullable: false),
 Amount = c.Double(nullable: false),
 Date = c.DateTime(nullable: false),
 })
 .PrimaryKey(t => t.OrderID);
 
 CreateTable(
 "dbo.Users",
 c => new
 {
 UserID = c.Int(nullable: false, identity: true),
 FirstName = c.String(),
 LastName = c.String(),
 })
 .PrimaryKey(t => t.UserID);
 
 }
 
 public override void Down()
 {
 DropTable("dbo.Users");
 DropTable("dbo.Orders");
 }
}

第三，数据库中对应的表被生成.

迁移第二个 DbContext(DataContext)

enable-migrations -ContextTypeName MultiDataContextMigrations.Models.UserDataContext -MigrationsDirectory:UserDataContextMigrations
Add-Migration -configuration MultiDataContextMigrations.UserDataContextMigrations.Configuration Initial
Update-Database -configuration MultiDataContextMigrations.UserDataContextMigrations.Configuration -Verbose

当你在程序包管理控制台窗口中执行上面命令，会有如下结果:

首先，会创建一个新的迁移文件夹和配置文件类.

internal sealed class Configuration : DbMigrationsConfiguration
{
 public Configuration()
 {
 AutomaticMigrationsEnabled = false;
 //Helps to migrate this DbContext to database
 MigrationsDirectory = @"UserDataContextMigrations";
 }
 protected override void Seed(MultiDataContextMigrations.Models.UserDataContext context)
 {
 // This method will be called after migrating to the latest version.
 }
}

第二，对应我们添加的迁移项，会生成一个类文件，显示更新的内容，本例中为创建表

public partial class Initial : DbMigration
{
 public override void Up()
 {
 CreateTable(
 "dbo.Roles",
 c => new
 {
 RoleID = c.Int(nullable: false, identity: true),
 RolesName = c.String(),
 })
 .PrimaryKey(t => t.RoleID);
 
 CreateTable(
 "dbo.Users",
 c => new
 {
 UserID = c.Int(nullable: false, identity: true),
 FirstName = c.String(),
 LastName = c.String(),
 })
 .PrimaryKey(t => t.UserID);
 
 }
 
 public override void Down()
 {
 DropTable("dbo.Users");
 DropTable("dbo.Roles");
 }
}

运行更新数据库命令之前,先注释掉生成user表的代码. 因为该表在上面的迁移代码中已经创建.

public partial class Initial : DbMigration
{
 public override void Up()
 {
 CreateTable(
 "dbo.Roles",
 c => new
 {
 RoleID = c.Int(nullable: false, identity: true),
 RolesName = c.String(),
 })
 .PrimaryKey(t => t.RoleID);
 
 //CreateTable(
 // "dbo.Users",
 // c => new
 // {
 // UserID = c.Int(nullable: false, identity: true),
 // FirstName = c.String(),
 // LastName = c.String(),
 // })
 // .PrimaryKey(t => t.UserID);
 
 }
 
 public override void Down()
 {
 DropTable("dbo.Users");
 DropTable("dbo.Roles");
 }
}

现在运行更新命令，会在上一个Dbcontex更新命令的基础上继续更新数据库。

__MigrationHistory表

这个表是迁移历史记录。我们简单看一下.

撤销/回滚 DbContexts 迁移

你可以使用下面一些列命令，对上面两个Dbcontex的迁移更新做回滚操作.

Update-Database -configuration MultiDataContextMigrations.DataContextMigrations.Configuration -TargetMigration:"201402141616393_Initial" -verbose
Update-Database -configuration MultiDataContextMigrations.UserDataContextMigrations.Configuration -TargetMigration:"201402141641408_Initial" -verbose

案例 2 :不同项目中的多个Dbcontex迁移

假设，你有如下图所示的两个Dbcontex，分别在不同的项目中.

在VS2012和VS2013中，执行下面的命令可以迁移在不同项目中的多个Dbcontex到同一数据库实例:

Syntax - EF Code First Migrations with Multiple DbContexts within different Projects

enable-migrations -ProjectName:<ProjectName> -MigrationsDirectory:<Migrations-Directory-Name>
add-migration <Migrations-Name> -ProjectName:<ProjectName>
update-database -ProjectName:<ProjectName> -verbose

迁移第一个DbContext(DataContext)

//migrating DataContext
enable-migrations -ProjectName:DAL -MigrationsDirectory:DataContextMigrations
add-migration InitialCreate -ProjectName:DAL
update-database -ProjectName:DAL -verbose

迁移第二个DbContext(UserDataContext)

//migrating UserDataContext
enable-migrations -ProjectName:MultiDataContextMigrations -MigrationsDirectory:UserDataContextMigrations
add-migration InitialCreate -ProjectName:MultiDataContextMigrations
update-database -ProjectName:MultiDataContextMigrations -verbose

撤销/回滚DbContexts Migrations

update-database -ProjectName:DAL -TargetMigration:"201401290826231_InitialCreate" -verbose
update-database -ProjectName:MultiDataContextMigrations -TargetMigration:"201401290836540_InitialCreate" -verbose

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