当我想使用db-migration更新数据库时如何防止数据丢失？

Question

When I was making update my database by using db-migration, I faced a problem that was

Automatic migration was not applied because it would result in data loss.

(I used System.ComponentModel.DataAnnotations such as [Required] and [StringLength(25)] for some properties. For example Title property.)

I know if I set the AutomaticMigrationDataLossAllowed to true and Update-Database -Force , my database will be update but my data will be removed and I'm going to prevent from it. I want to protect my data.

I've used Entity Framework 6.x

How can I solve this problem?

Configuration class:

namespace Jahan.Blog.Web.Mvc.Migrations
{
   using System;
   using System.Data.Entity;
   using System.Data.Entity.Migrations;
   using System.Linq;

   internal sealed class Configuration 
    : DbMigrationsConfiguration<Jahan.Blog.Web.Mvc.Models.JahanBlogDbContext>
   {
       public Configuration()
       {
           AutomaticMigrationsEnabled = true;
           AutomaticMigrationDataLossAllowed = false;
       }

       protected override void Seed(Jahan.Blog.Web.Mvc.Models.JahanBlogDbContext context)
       {

       }
   }
}

Initial class:

namespace Jahan.Blog.Web.Mvc.Migrations
{
   using System;
   using System.Data.Entity.Migrations;

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

       public override void Down()
       {
       }
   }
}

My DbContext:

namespace Jahan.Blog.DataAccess
{
   public class JahanBlogDbContext : IdentityDbContext<User, Role, int, UserLogin, UserRole,    UserClaim>
   {
       public JahanBlogDbContext()
           : base("name=JahanBlogDbConnectionString")
       {

       }
       protected override void OnModelCreating(DbModelBuilder modelBuilder)
       {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        modelBuilder.Entity<Comment>().HasRequired(t => t.Article).WithMany(t => t.Comments).HasForeignKey(d => d.ArticleId).WillCascadeOnDelete(true);
        base.OnModelCreating(modelBuilder);

        modelBuilder.Entity<User>().ToTable("User");
        modelBuilder.Entity<Role>().ToTable("Role");
        modelBuilder.Entity<UserRole>().ToTable("UserRole");
        modelBuilder.Entity<UserLogin>().ToTable("UserLogin");
        modelBuilder.Entity<UserClaim>().ToTable("UserClaim");
       }
       // ... codes ....
   }
}

Answer 1

You can add sql to fix the data in a way that is acceptable to you. You need to ensure that the alter statements generated BY EF do not cause data loss.

Use the Sql method in a migration to run your own sql:

public override void Up()
{
    //Add this to your migration...
    Sql("UPDATE dbo.Table SET Name = LEFT(Name, 25) WHERE LEN(Name) > 25")

    //...before the code generated by EF
    AlterColumn("dbo.Table", "Name ", c => c.String(nullable: false, maxLength: 25));
}