更新实体框架中的特定字段

Question

我有一个模特:

public class VR
{
    [Key]
    public int ID { get; set; }
    public string FullName { get; set; }
    public string CreatedBy { get; set; }
    public DateTime? Created { get; set; }
    public string ModifiedBy { get; set; }
    public DateTime? Modified { get; set; }
}

我的控制器编辑功能:

    // POST: VRs/Edit/5
    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Edit(VR vR)
    {
        if (ModelState.IsValid)
        {
            var Result = (from c in _context.MyVR.Where(c => c.ID == vR.ID) select c).Single();

            vR.Created = Result.Created;
            vR.CreatedBy = Result.CreatedBy;
            vR.ModifiedBy = User.Identity.Name;
            vR.Modified = DateTime.Now;

            _context.Update(vR);
            _context.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(vR);
    }

我得到以下错误:

无法跟踪实体类型"UNTest.ViewModels.VR"的实例,因为已经跟踪了具有相同密钥的此类型的另一个实例.对于新实体,请考虑使用IIdentityGenerator生成唯一键值.

Answer 1

您可以附加实体以避免从DB加载(保存性能)并仅更新所需的字段.

当您从DB(Result)加载实例并使用相同的Id(vR)跟踪另一个实例导致异常时,这也可以避免代码问题.

// POST: VRs/Edit/5
    [HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Edit(VR vR)
    {
        if (ModelState.IsValid)
        {
            //Attach the instance so that we don't need to load it from the DB
            _context.MyVR.Attach(vR);

            vR.ModifiedBy = User.Identity.Name;
            vR.Modified = DateTime.Now;

            //Specify the fields that should be updated.
            _context.Entry(vR).Property(x => x.ModifiedBy).IsModified = true;
            _context.Entry(vR).Property(x => x.Modified).IsModified = true;

            _context.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(vR);
    }

另一种指定不应更新的字段的方法.

// POST: VRs/Edit/5
        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Edit(VR vR)
        {
            if (ModelState.IsValid)
            {
                //Attach the instance so that we don't need to load it from the DB
                _context.Entry(vR).State = EntityState.Modified; 

                vR.ModifiedBy = User.Identity.Name;
                vR.Modified = DateTime.Now;

                //Specify the fields that should not be updated.
                _context.Entry(vR).Property(x => x.Created).IsModified = false;
                _context.Entry(vR).Property(x => x.CreatedBy).IsModified = false;

                _context.SaveChanges();
                return RedirectToAction("Index");
            }
            return View(vR);
        }

如果您使用视图模型,则可以使用new运算符创建数据模型并复制要更新的字段:

// POST: VRs/Edit/5
            [HttpPost]
            [ValidateAntiForgeryToken]
            public IActionResult Edit(VRViewModel vRVM)
            {
                if (ModelState.IsValid)
                {
                    VR vR = new VR();
                    //Attach the instance so that we don't need to load it from the DB
                    _context.MyVR.Attach(vR);

                    //Set the Id for your model.
                    vR.Id = vRVM.Id;
                    //Let's say you also want to update this field from the VM
                    vR.FullName = vRVM.FullName;

                    vR.ModifiedBy = User.Identity.Name;
                    vR.Modified = DateTime.Now;

                    //Specify the fields that should be updated.
                    _context.Entry(vR).Property(x => x.ModifiedBy).IsModified = true;
                    _context.Entry(vR).Property(x => x.Modified).IsModified = true;
                    _context.Entry(vR).Property(x => x.FullName).IsModified = true;

                    _context.SaveChanges();
                    return RedirectToAction("Index");
                }
                //create your new view model and return it. For demonstration purpose, I return the same view model, in your real code, you can adjust it.
                return View(vRVM);
            }