Question

I’ve checked this question and it seems to be related to what I need, but does not answer it exactly.

I have an entity (Sql Compact using EF Code First via MVC3- if that wasn’t clear from the title) for an “Issue” (generic issue tracking, just for my own education understanding of how MVC3 works). The Issue class has a CreatedBy property (Int reference to a User who Created the Issue) and a CreatedDate property (DateTime). When I use the scaffolded code to update (modified only to prevent some updated date fields from being modified by the user):

        if (ModelState.IsValid)
        {
            issue.LastActivity = (DateTime?)DateTime.Now.Date;
            if (issue.ClosedBy != null) issue.ClosedDate = (DateTime?)DateTime.Now.Date;
            startingIssue = null;
            db.Entry(issue).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }

I receive the error mentioned in the linked question (conversion of a datetime2 data type to a datetime data type etc., etc.,)

When I step through the code, it appears my CreatedBy and CreatedDate properties are not contained in the instance of issue that the controller is passing around. When I try to fix that by grabbing another copy of the issue from the db, and updating those to values:

        var startingIssue = db.Issues.Find(issue.IssueId);
        if (ModelState.IsValid)
        {
            if (issue.CreatedBy != startingIssue.CreatedBy) issue.CreatedBy = startingIssue.CreatedBy;
            if (issue.CreatedDate != startingIssue.CreatedDate) issue.CreatedDate = startingIssue.CreatedDate;
            issue.LastActivity = (DateTime?)DateTime.Now.Date;
            if (issue.ClosedBy != null) issue.ClosedDate = (DateTime?)DateTime.Now.Date;
            startingIssue = null;
            db.Entry(issue).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }

I get the concurrency violation: An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

So, how do I get EF to see the date which is already set in the DB (so it doesn’t try to update the CreatedDate to 1/1/0001) without violating concurrency?

Edit
Okay… I found it. I was, apparently, looking for @Html.HiddenFor(model => model.[property]) and adding the editor to the view anyway. That seems a little silly and round-about to me, but it does work without having to add custom code to detach one object and substitute an updated one.

Answer 1

The short answer is that you’ve already loaded the entity into the context with the Find and you cannot later attach another one.

You are left with two options:

• Detach the first instance, then attach the second
• Copy the fields from the second instance to the first

I’ll share code for the first option. First, add a Detach method to your DbContext implementation:

public void Detach(object entity)
{
    var objectContext = ((IObjectContextAdapter)this).ObjectContext;
    objectContext.Detach(entity);
}

Then call Detach instead of setting the variable to null

var startingIssue = db.Issues.Find(issue.IssueId);
if (ModelState.IsValid)
{
    if (issue.CreatedBy != startingIssue.CreatedBy) issue.CreatedBy = startingIssue.CreatedBy;
    if (issue.CreatedDate != startingIssue.CreatedDate) issue.CreatedDate = startingIssue.CreatedDate;
    issue.LastActivity = (DateTime?)DateTime.Now.Date;
    if (issue.ClosedBy != null) issue.ClosedDate = (DateTime?)DateTime.Now.Date;

    // startingIssue = null;
    db.Detach(startingIssue);

    db.Entry(issue).State = EntityState.Modified;
    db.SaveChanges();
    return RedirectToAction("Index");
}