更新实体框架中的特定字段

Question

我有一个 model：

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Edit function：

    // 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);
    }

我收到以下错误：

无法跟踪实体类型“ UNTest.ViewModels.VR ”的实例，因为已跟踪具有相同键的该类型的另一个实例。 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 ）加载实例并跟踪具有相同Id（ vR ）的另一个实例时，这也可以避免代码问题，从而导致异常。

// 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);
            }

Answer 2

您已从上下文中检索数据实体，以便跟踪它。 然后，您无法在同一上下文中跟踪具有相同ID的另一个实体。

更改POST方法以从视图模型更新数据模型，并保存数据模型

[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)
           .FirstOrDefault(); // use FirstOrDefault() to prevent an exception if the user changes you input for the ID.
        if (result != null)
        {
           result.FullName = vR.FullName;
           result.ModifiedBy = User.Identity.Name;
           result.Modified = DateTime.Now;
           _context.Update(result);
           _context.SaveChanges();
            return RedirectToAction("Index");
        }
    return View(vR);
}

Answer 3

我参加聚会迟到了，但是。 从上面的答案复制下面的代码。

// 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;
        _context.MyVR.Attach(vR).State = EntityState.Modified;
        //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);
}