internal sealed class Configuration : DbMigrationsConfiguration<MyDbContext>
{
    ...
}

public partial class Init: DbMigration
{
    public override void Up()
    {
        ...
    }

    public override void Down()
    {
        ...
    }
}

public Configuration()
{
    AutomaticMigrationsEnabled = true;
    MigrationsDirectory = @"MigrationsContext1";
}