EntityFrameworkCore使用Migrations自動更新數據庫

系統環境：Win10
IDE：VS2017 RC4
.netcore版本：1.1

一、新建ASP.NET Core WebApi項目

二、引用Microsoft.EntityFrameworkCore.Sqlite

使用VS Nuget工具，添加對Microsoft.EntityFrameworkCore.Sqlite庫的引用，如使用其他數據庫，添加相對應的引用即可。

三、使項目支持dotnet ef工具以使用Migrations

手動修改項目csproj文件

在ItemGroup.DotNetCliToolReference節點添加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。
並在ItemGroup.PackageReference節點添加對Microsoft.EntityFrameworkCore.Design的引用，因為使用dotnet ef migrations add命令需要該引用。

  <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>

手動添加的原因是我在Nuget添加Microsoft.EntityFrameworkCore.Tools.DotNet時，報了一個錯，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到項目目錄下（非Solution目錄），執行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獨角獸，說明工具引用成功。

四、創建實例DbContext

新建DbContext以及相關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將DbContext注冊為服務

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

五、使用Migrations

新建數據庫初始化類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();//獲取未應用的Migrations，不必要，MigrateAsync方法會自動處理
            await context.Database.MigrateAsync();//根據Migrations修改/創建數據庫
        }
    }
}

在Startup.cs的Configure方法中，添加參數BloggingContext context，netcore會使用DI將DbContext注入到Configure方法，並添加對DbInitializer的調用。

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

使用dotnet ef命令創建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'

執行成功后，可以看到項目目錄下多了個文件夾Migrations，里面就是自動生成的Migrations文件。

創建更新Migrations的命令跟創建初始的一樣，只是最后一個參數自己定義即可。

最后執行代碼，成功創建了新的數據庫，並且里面有一個表__EFMigrationsHistory，是自動生成的用來記錄Migrations記錄。

修改Blog類，添加一個字段public string NewField { get; set; }，在命令行執行命令dotnet ef migrations add NewField，然后運行程序，可以看到數據庫已經更新，__EFMigrationsHistory表也多了一條對應的記錄。

其他說明

因為我習慣使用VS，但是構建這個項目的過程中，需要使用到命令行，還需要手動修改csproj文件，所以這篇文章的操作過程有可能點亂，我自己在寫Demo的時候，也是多次重啟VS使VS能夠加載最新的項目。

Demo源碼，測試通過：Github

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