[英]Determining the range of value for a field in database by using db-migration
[英]How to prevent data loss when I want to update database by using db-migration?
当我使用db-migration更新我的数据库时,我遇到了一个问题
Automatic migration was not applied because it would result in data loss.
(对于某些属性,我使用了System.ComponentModel.DataAnnotations
,如[Required]
和[StringLength(25)]
。例如Title
属性。)
我知道如果我将AutomaticMigrationDataLossAllowed
设置为true
并且Update-Database -Force
,我的数据库将会更新,但我的数据将被删除,我将阻止它。 我想保护我的数据。
我使用过Entity Framework 6.x.
我怎么解决这个问题?
配置类:
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)
{
}
}
}
初级课程:
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()
{
}
}
}
我的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 ....
}
}
您可以添加sql以您可接受的方式修复数据。 您需要确保BY EF生成的alter语句不会导致数据丢失。
在迁移中使用Sql
方法来运行自己的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));
}
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