EntityFrameworkCore使用Migrations自动更新数据库

系统环境：Win10
IDE：VS2017 RC4
.netcore版本：1.1

一、新建ASP.NET Core WebApi项目

二、引用Microsoft.EntityFrameworkCore.Sqlite

使用VS Nuget工具，添加对Microsoft.EntityFrameworkCore.Sqlite库的引用，如使用其他数据库，添加相对应的引用即可。

三、使项目支持dotnet ef工具以使用Migrations

手动修改项目csproj文件

在ItemGroup.DotNetCliToolReference节点添加Microsoft.EntityFrameworkCore.Tools.DotNet工具的引用，注意版本1.0.0-msbuild3-final，VS2017 RC4 用的是MSBuild格式。
并在ItemGroup.PackageReference节点添加对Microsoft.EntityFrameworkCore.Design的引用，因为使用dotnet ef migrations add命令需要该引用。

  <ItemGroup>
    ······
    <PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="1.1.0" />
    ······
  </ItemGroup>
  <ItemGroup>
    ······
    <DotNetCliToolReference Include="Microsoft.EntityFrameworkCore.Tools.DotNet" Version="1.0.0-msbuild3-final" />
  </ItemGroup>

手动添加的原因是我在Nuget添加Microsoft.EntityFrameworkCore.Tools.DotNet时，报了一个错，可能是VS2017 RC版本的BUG：

Package 'Microsoft.EntityFrameworkCore.Tools.DotNet 1.1.0-preview4-final' has a package type 'DotnetCliTool' that is not supported by project 'EntityFrameworkCoreMigrationsDemo'.

CMD命令行cd到项目目录下（非Solution目录），执行dotnet build，dotnet ef

C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo>dotnet build
Microsoft (R) Build Engine version 15.1.545.13942
Copyright (C) Microsoft Corporation. All rights reserved.

Startup.cs(45,13): warning CS4014: Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call. [C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo.csproj]
  EntityFrameworkCoreMigrationsDemo -> C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo\bin\Debug\netcoreapp1.0\EntityFrameworkCoreMigrationsDemo.dll

Build succeeded.

Startup.cs(45,13): warning CS4014: Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call. [C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo.csproj]
    1 Warning(s)
    0 Error(s)

Time Elapsed 00:00:04.76

C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo>dotnet ef

                     _/\__
               ---==/    \\
         ___  ___   |.    \|\
        | __|| __|  |  )   \\\
        | _| | _|   \_/ |  //|\\
        |___||_|       /   \\\/\\

Entity Framework Core .NET Command Line Tools 1.0.0-msbuild3-final

Usage: dotnet ef [options] [command]

Options:
  --version        Show version information
  -h|--help        Show help information
  -v|--verbose     Show verbose output.
  --no-color       Don't colorize output.
  --prefix-output  Prefix output with level.

Commands:
  database    Commands to manage the database.
  dbcontext   Commands to manage DbContext types.
  migrations  Commands to manage migrations.

Use "dotnet ef [command] --help" for more information about a command.

可以看到EF独角兽，说明工具引用成功。

四、创建实例DbContext

新建DbContext以及相关Models

using Microsoft.EntityFrameworkCore;
using System.Collections.Generic;

namespace EntityFrameworkCoreMigrationsDemo.Data
{
    public class BloggingContext : DbContext
    {
        public DbSet<Blog> Blogs { get; set; }
        public DbSet<Post> Posts { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseSqlite("Filename=./Blogging.db");
        }
    }
    public class Blog
    {
        public int BlogId { get; set; }
        public string Url { get; set; }

        public List<Post> Posts { get; set; }
    }
    public class Post
    {
        public int PostId { get; set; }
        public string Title { get; set; }
        public string Content { get; set; }

        public int BlogId { get; set; }
        public Blog Blog { get; set; }
    }
}

在Startup将DbContext注册为服务

namespace EntityFrameworkCoreMigrationsDemo
{
    public class Startup
    {
        ······
        public void ConfigureServices(IServiceCollection services)
        {
            ······
            services.AddDbContext<BloggingContext>();
        }
        ······
    }
}

五、使用Migrations

新建数据库初始化类InitializeAsync.cs

using Microsoft.EntityFrameworkCore;
using System.Threading.Tasks;

namespace EntityFrameworkCoreMigrationsDemo.Data
{
    public class DbInitializer
    {
        public async Task InitializeAsync(BloggingContext context)
        {
            //var migrations = await context.Database.GetPendingMigrationsAsync();//获取未应用的Migrations，不必要，MigrateAsync方法会自动处理
            await context.Database.MigrateAsync();//根据Migrations修改/创建数据库
        }
    }
}

在Startup.cs的Configure方法中，添加参数BloggingContext context，netcore会使用DI将DbContext注入到Configure方法，并添加对DbInitializer的调用。

    public class Startup
    {
        ······
        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, BloggingContext context)
        {
            ······
            new DbInitializer().InitializeAsync(context);
        }
    }

使用dotnet ef命令创建Migrations：dotnet ef migrations add [NAME]

C:\WorkSpacesC\DotNetCore\EntityFrameworkCoreMigrationsDemo\EntityFrameworkCoreMigrationsDemo>dotnet ef migrations add Initial

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:02.32
Done. To undo this action, use 'ef migrations remove'

执行成功后，可以看到项目目录下多了个文件夹Migrations，里面就是自动生成的Migrations文件。

创建更新Migrations的命令跟创建初始的一样，只是最后一个参数自己定义即可。

最后执行代码，成功创建了新的数据库，并且里面有一个表__EFMigrationsHistory，是自动生成的用来记录Migrations记录。

修改Blog类，添加一个字段public string NewField { get; set; }，在命令行执行命令dotnet ef migrations add NewField，然后运行程序，可以看到数据库已经更新，__EFMigrationsHistory表也多了一条对应的记录。

其他说明

因为我习惯使用VS，但是构建这个项目的过程中，需要使用到命令行，还需要手动修改csproj文件，所以这篇文章的操作过程有可能点乱，我自己在写Demo的时候，也是多次重启VS使VS能够加载最新的项目。

Demo源码，测试通过：Github

参考：
.NET Core - New Database Create your model
Walkthrough: Entity Framework Core 1.1 with Migrations