项目更新时，CreatedAt字段变为空值

Question

I'm facing a problem when updating items. When a new item is created, the CreatedAt value is populated with the current time & date. When I edit an item, the CreatedAt value immediately gets set to null. The UpdatedAt value gets set correctly but CreatedAt is set to null in the database.

Here's the ItemViewModel class:

public class ItemViewModel
{
    public int ItemId { get; set; }
    public string ItemName { get; set; }
    public int UserId { get; set; }
    public string CreatedBy { get; set; }
    public DateTime? CreatedAt { get; set; }
    public string UpdatedBy { get; set; }
    public DateTime? UpdatedAt { get; set; }

    public virtual User User { get; set; }
}

This is the update method I've used:

public void UpdateItem(ItemViewModel viewModel)
{
    item.UpdatedBy = WebSecurity.CurrentUserName;
    item.UpdatedAt = DateTime.Now;
    db.SaveChanges();
}

The following create method populates the CreatedAt value correctly:

public int CreateItem(ItemViewModel viewModel)
{
    var item = new Item();
    viewModel.CopyToItem(item); 

    item.UserId = WebSecurity.CurrentUserId;
    item.CreatedBy = WebSecurity.CurrentUserName;
    item.CreatedAt = DateTime.Now;

    db.Items.Add(item);
    db.SaveChanges();
    return item.ItemId;
}

Is there something I've missed out?

Answer 1

You should check for the Entry is Modified or not using EntityState as:

public void UpdateItem(ItemViewModel viewModel)
{
    var item=db.Table.where(x=>x.ID==viewModel.ID).FirstOrDefault();
    item.UpdatedBy = WebSecurity.CurrentUserName;
    item.UpdatedAt = DateTime.Now;
    db.Entry(item).state=EntityState.Modified;
    db.SaveChanges();
}

Answer 2

In this method:

public void UpdateItem(ItemViewModel viewModel)
{
    item.UpdatedBy = WebSecurity.CurrentUserName;
    item.UpdatedAt = DateTime.Now;
    db.SaveChanges();
}

Where is viewModel coming from? (And, come to think of it, what is item ?) Is this an action being invoked by a client-side form? I suspect what's happening here is that the client-side form doesn't include the data for CreatedAt , so the model binder leaves the value as null when building the parameter for this method.

The model binder isn't "smart enough" to go fetch the current persisted state of the model and merge that with what comes from the form. It only has what comes from the form. So you essentially have two options:

1. Persist to the page's form elements all data required to accurately re-construct the model.
2. Persist to the page's form elements at least an identifier for the model and in server-side code use that identifier to re-fetch the state of the model from the persisted data.

For the first option, you'll need to include CreatedAt . Which could be something as simple as:

@Html.HiddenFor(model => model.CreatedAt)

Answer 3

Try like this

 var data= db.Items.where(x=>x.ID==viewModel.ID).SingleOrDefault();
    foreach(db.Items in data)
    {
        item.UpdatedBy = WebSecurity.CurrentUserName;
        item.UpdatedAt = DateTime.Now;
    }
     db.SaveChanges();