entity-framework - EF 核心 : Add custom LINQ in Up method of Migration class

Question

我想在迁移的 Up 方法中执行一些 LINQ。问题是我不知道如何获取 DbContext 实例？

这是由 migrations add 生成的代码:

public partial class MyTableAddFieldTitle : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.AddColumn<string>(
            name: "Title",
            table: "MyTable",
            nullable: true);
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.DropColumn(
            name: "Title",
            table: "MyTable");
    }
}

我想在 Up 方法中添加类似的东西:

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.AddColumn<string>(
        name: "Title",
        table: "MyTable",
        nullable: true);

    var context = ?????;

    //Actual code is much more complicated, but the principle is the same.
    foreach (var item in context.Set<DbMyTable>())
      item.Title = item.SomeStringColumn;
    context.SaveChanges();
}

问题是如何获取上下文实例？我在构造函数中尝试使用 DI:

protected MyTableAddFieldTitle(MyContext context)
{
}

但我得到错误:

MissingMethodException: No parameterless constructor defined for this object. System.RuntimeTypeHandle.CreateInstance(RuntimeType type, bool publicOnly, ref bool canBeCached, ref RuntimeMethodHandleInternal ctor)

Answer 1

我找到了解决方案。

在 Startup 类中我定义了静态变量:

public static Func<MyContext> ContextFactory;

在 Startup 类的构造函数中，我分配了变量:

public Startup(IHostingEnvironment env, IConfiguration config)
{
    MyContext GetContext(IConfiguration configuration, IHostingEnvironment environment)
    {
        var builder = new DbContextOptionsBuilder<MyContext>();
        builder.UseSqlServer(configuration["ConnectionStrings:Web"], b => b.MigrationsAssembly("Web.Hosting"));
        if (environment.IsDevelopment())
            builder.EnableSensitiveDataLogging();
        return new MyContext(builder.Options);
    }

    ContextFactory = () => GetContext(config, env);
}

然后在迁移中，我只需调用 ContextFactory:

var context = Startup.ContextFactory();
context.Set<DbMyTable>().Where(....

为避免错误字段不存在，我创建了 2 个迁移文件(dotnet ef migrations add)。
首先添加字段:

protected override void Up(MigrationBuilder migrationBuilder)
{
    migrationBuilder.AddColumn<string>(
        name: "Title",
        table: "MyTable",
        nullable: true);
}

第二个(空)执行查询:

protected override void Up(MigrationBuilder migrationBuilder)
{
    var context = Startup.ContextFactory();
    context.Set<DbMyTable>().Where(....
}