EF MVC 关联实体 更新操作 代码为什么要这么写?

Question

这是从 EF自动生成的edit上修改的.关于文字类型的操作
ArticleCategory:

public partial class ArticleCategory : Entity
    {
        public ArticleCategory()
        {
            this.Article = new HashSet<Article>();
        }

        [Display(Name = "分类名称")]
        [StringLength(225, ErrorMessage = "内容太长", MinimumLength = 1)]
        public string Name { get; set; }

        public virtual ArticleCategory PId { get; set; }

        public virtual ICollection<Article> Article { get; set; }

        public virtual ICollection<ArticleCategory> Children { get; set; }
    }

在修改的过程中,发现如下写法,没有办法修改 PId

         public async Task Edit(ArticleCategory articleCategory,string pId)
        {
            if (pId!=null && pId!="0")
            {
                articleCategory.PId = await FindAsync(int.Parse(pId)); 
            }
            //ArticleCategory art = articleCategory;
            //articleCategory = await FindAsync(art.Id);
            //articleCategory.PId = art.PId;
            //articleCategory.Name = art.Name;
            //articleCategory.IsDelete = art.IsDelete;
            //articleCategory.OrderNo = art.OrderNo;
            //articleCategory.Remarks = art.Remarks;
            //articleCategory.UpdateTime = art.UpdateTime;

            db.Entry(articleCategory).State = EntityState.Modified;
            await SaveChangesAsync();
        }

写成这样会报错:

         public async Task Edit(ArticleCategory articleCategory,string pId)
        {

            if (pId!=null && pId!="0")
            {
                articleCategory.PId = await FindAsync(int.Parse(pId)); 
            }
            ArticleCategory art = articleCategory;
            articleCategory = await FindAsync(art.Id);
            articleCategory = art;
            //articleCategory.PId = art.PId;
            //articleCategory.Name = art.Name;
            //articleCategory.IsDelete = art.IsDelete;
            //articleCategory.OrderNo = art.OrderNo;
            //articleCategory.Remarks = art.Remarks;
            //articleCategory.UpdateTime = art.UpdateTime;

            db.Entry(articleCategory).State = EntityState.Modified;
            await SaveChangesAsync();
        }

Attaching an entity of type 'DataModel.ArticleCategory' failed because another entity of the same type already has the same primary key value. This can happen when using the 'Attach' method or setting the state of an entity to 'Unchanged' or 'Modified' if any entities in the graph have conflicting key values. This may be because some entities are new and have not yet received database-generated key values. In this case use the 'Add' method or the 'Added' entity state to track the graph and then set the state of non-new entities to 'Unchanged' or 'Modified' as appropriate.

只有这样写了才行.

         public async Task Edit(ArticleCategory articleCategory,string pId)
        {
            if (pId!=null && pId!="0")
            {
                articleCategory.PId = await FindAsync(int.Parse(pId)); 
            }
            ArticleCategory art = articleCategory;
            articleCategory = await FindAsync(art.Id);
            articleCategory.PId = art.PId;
            articleCategory.Name = art.Name;
            articleCategory.IsDelete = art.IsDelete;
            articleCategory.OrderNo = art.OrderNo;
            articleCategory.Remarks = art.Remarks;
            articleCategory.UpdateTime = art.UpdateTime;

            db.Entry(articleCategory).State = EntityState.Modified;
            await SaveChangesAsync();
        }

已傻,求解答~~~~

Answer 1

public virtual ArticleCategory PId { get; set; }
没必要定义virtual