c# - MVC EF - System.Data.Entity.Infrastruct.DbUpdateException

Question

我有两个型号:用户资料

public class UserProfile
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ApplicationUser ApplicationUser { get; set; }
    public virtual ICollection<UserPost> UserPost { get; set; }
}

用户帖子:

public class UserPost
{  
   [Key]
   [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public string Id { get; set; }
   public string PostTitle { get; set; }
   public virtual UserProfile UserProfile { get; set; }
   public string Contents { get; set; }
}

UserProfile 与 AspNetUser 具有一对一的关系。

UserProfile 与 UserPost 具有一对多关系。

当我在 Controller 的 Create 方法中将 UserPost 添加到数据库时，我收到 System.Data.Entity.Infrastruct.DbUpdateException

    [HttpPost]
    [ValidateAntiForgeryToken]
    [Authorize]
    public ActionResult Create([Bind(Include = "PostTitle, Content")] UserPost post)
    {
        if (ModelState.IsValid)
        {

            UserManager = new UserManager<ApplicationUser>(new UserStore<ApplicationUser>(db));
            var user = UserManager.FindById(User.Identity.GetUserId());
            post.UserProfile = user.UserProfile;
            db.UserPosts.Add(post);
            db.SaveChanges();//error here
            return RedirectToAction("Index");
        }

        return View(post);
    }

System.Data.Entity.Infrastructure.DbUpdateException occurred HResult=0x80131501 Message=An error occurred while saving entities that do not expose foreign key properties for their relationships. The EntityEntries property will return null because a single entity cannot be identified as the source of the exception. Handling of exceptions while saving can be made easier by exposing foreign key properties in your entity types. See the InnerException for details.

Inner Exception 1: UpdateException: An error occurred while updating the entries. See the inner exception for details.

Inner Exception 2: SqlException: Cannot insert the value NULL into column 'Id', table 'aspnet-project12017.dbo.UserPosts'; column does not allow nulls. INSERT fails. The statement has been terminated.

Answer 1

[DatabaseGenerated(DatabaseGeneratedOption.Identity)]意味着标有此类属性的列预期具有从 DBMS 自动生成的值，并且作为一般约定，EF 将 string 和 uniqueidentifier (GUID) 列视为非自动生成的列递增，因此这些列值必须以另一种方式分配(即通过构造函数分配)。

解决主键自增标识列为空问题的常用方法是将其设置为数字数据类型(例如int):

public class UserPost
{  
   [Key]
   [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
   public int Id { get; set; } // change to numeric data type
   public string PostTitle { get; set; }
   public virtual UserProfile UserProfile { get; set; }
   public string Contents { get; set; }
}

然后，在执行迁移时，执行以下步骤:

1. 有时 EF 默认情况下期望您尝试插入到标识列中。如果发生这种情况，请暂时将 StoreGeneratePattern.None 和 DatabaseGenerationOption.None 设置为 UserPost.Id:

   protected void OnModelCreating(DbModelBuilder modelBuilder)
   {
       // other entities
   
       modelBuilder.Entity<UserPost>()
          .HasKey(p => p.Id)
          .Property(p => p.Id)
          .StoreGeneratedPattern = StoreGeneratedPattern.None;
   
      // other entities
   }
   

2. 确保模型已修改为包含实际的 Id 属性(需要时尝试删除数据库中的 UserPost 表)。

   <

3. 通过程序包管理器控制台运行迁移，如下所示:

   Add-Migration "Set Id as Identity Specification"
   

4. 在 DbMigration 生成的类中，您将看到两个方法:Up() 和 Down()。修改 Up() 部分以设置 int 数据类型(并在需要时重新运行迁移过程):

   public override void Up()
   {
       CreateTable(
             "dbo.UserPost",
             c => new
             {
                 Id = c.Int(nullable: false, identity: true),
                 PostTitle = c.String(),
                 Contents = c.String(),
             })
             .PrimaryKey(t => t.Id);                
   }
   

如果您仍希望字符串 Id 列作为 UserPost 的主键，则需要将其设置为 [DatabaseGenerate(DatabaseGenerateOption.None)] 并从类构造函数手动分配值(请注意，生成的值必须是唯一的，否则 EF 会告诉您“无法在对象中插入重复的键”):

public class UserPost
{
    public UserPost()
    {
        Id = [[autogenerated value here]];
    }

    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    public string Id { get; set; }

    // other properties
}

相关问题:

MVC5 EF6 Seed Method: Cannot insert NULL into column 'Id' table AspNetUsers

Cannot insert the value NULL into column 'Id' (Database first)

Code first - Cannot insert the value NULL into column 'Id'

Entity Framework Cannot insert the value NULL into column Identity Specification set to No