尝试 cascade-delete 时出错

Question

我在尝试从数据库中删除项目时收到以下错误消息：

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

我已经阅读了很多关于这个问题的主题，但其中 none 似乎有所帮助（或者我不太理解它们）。

我的模型是：

public class ARDOperation
    {    
        [Key]                
        public int ARD { get; set; }

        [Required]               
        public virtual ICollection<Act> Actions { get; set; }

        public ARDOperation()
        {
            this.Actions = new List<Act>();
        }

    }
public class Act
{                      
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int ARDID { get; set; }
    public int ARDOperationId { get; set; }

    [ForeignKey("ARDOperationId")]        
    public virtual ARDOperation ARDOperation { get; set; }

    public string Data { get; set; }

    [EnumDataType(typeof(ARDState))]
    public ARDState State { get; set; }
}

我也定义了一个流利的API:

    public class ARDOperationDBContext : DbContext
    {            
        public DbSet<ARDOperation> ARDOperation { get; set; }
        //public DbSet<Act> Act { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Act>()
            .HasRequired(t => t.ARDOperation)
            .WithMany(t => t.Actions)
            .HasForeignKey(d => d.ARDOperationId)
            .WillCascadeOnDelete(true);                

            //modelBuilder.Entity<ARDOperation>()

        }

控制器中的方法：

        internal void RemoveAction(int ARDID)
        {
        var op = ARDOperationDB.ARDOperation.Find(ARDID);
        if (op != null)
        {
            //will not remove the "idle" action
            if (op.Actions.Count > 1)                                
            {
                Act act = op.Actions.ElementAt(1);                                        
                op.Actions.Remove(act);                    
                //ARDOperationDB.Entry(op).State = EntityState.Modified;
                ARDOperationDB.SaveChanges();                                                           
            }                
        }
    }

我尝试使用 code-first 方法将 "ARDOperationId" 属性定义为可为空（int？），但我没有收到任何错误，但是 child的数据仍然保留在数据库中。

我认为我遗漏了与访问 Act 模型相关的内容。

感谢任何帮助，尤瓦尔.

Answer 1

看看来自 EF 大师关于删除方法的[这个答案][1]。

EntityCollection.Remove(childEntity) marks the relationship between parent and childEntity as Deleted. If the childEntity itself is deleted from the database and what exactly happens when you call SaveChanges depends on the kind of relationship between the two:

If the relationship is optional, i.e. the foreign key that refers from the child to the parent in the database allows NULL values, this foreign will be set to null and if you call SaveChanges this NULL value for the childEntity will be written to the database (i.e. the relationship between the two is removed). This happens with a SQL UPDATE statement. No DELETE statement occurs.

If the relationship is required (the FK doesn't allow NULL values) and the relationship is not identifying (which means that the foreign key is not part of the child's (composite) primary key) you have to either add the child to another parent or you have to explicitly delete the child (with DeleteObject then). If you don't do any of these a referential constraint is violated and EF will throw an exception when you call SaveChanges - the infamous "The relationship could not be changed because one or more of the foreign-key properties is non-nullable" exception or similar.

If the relationship is identifying (it's necessarily required then because any part of the primary key cannot be NULL) EF will mark the childEntity as Deleted as well. If you call SaveChanges a SQL DELETE statement will be sent to the database. If no other referential constraints in the database are violated the entity will be deleted, otherwise an exception is thrown.

[1]: Entity Framework .Remove() vs. .DeleteObject()

Answer 2

所以我阅读了很多关于这个主题的文章。克里斯在这里的回复非常好，对我的理解很有帮助：Link

但真正帮助我的是这里的小代码示例：Solution。

“[Key, ForeignKey("Order"), Column(Order = 1)]”部分真的很成功。

非常感谢！

尝试 cascade-delete 时出错

Error while trying to cascade-delete

c#

asp.net

entity-framework

foreign-key-relationship