Question

I have a many-to-many relationship between Issues and Scopes in my EF Context. In ASP.NET MVC, I bring up an Edit form that allows the user to edit a particular Issue. At the bottom of the form, is a list of checkboxes that allow them to select which scopes apply to this issue. When editing an issue, it likely will always have some Scopes associated with it already–these boxes will be checked already. However, the user has the opportunity to check more scopes or remove some of the currently checked scopes. My code looked something like this to save just the Issue:

            using (var edmx = new MayflyEntities())
            {
                Issue issue = new Issue { IssueID = id, TSColumn = formIssue.TSColumn };
                edmx.Issues.Attach(issue);

                UpdateModel(issue);

                if (ModelState.IsValid)
                {
                    //if (edmx.SaveChanges() != 1) throw new Exception("Unknown error. Please try again.");
                    edmx.SaveChanges();
                    TempData["message"] = string.Format("Issue #{0} successfully modified.", id);
                }
            }

So, when I try to add in the logic to save the associated scopes, I tried several things, but ultimately, this is what made the most sense to me:

            using (var edmx = new MayflyEntities())
            {
                Issue issue = new Issue { IssueID = id, TSColumn = formIssue.TSColumn };
                edmx.Issues.Attach(issue);

                UpdateModel(issue);

                foreach (int scopeID in formIssue.ScopeIDs)
                {
                    var thisScope = new Scope { ID = scopeID };
                    edmx.Scopes.Attach(thisScope);
                    thisScope.ProjectID = formIssue.ProjectID;
                    if (issue.Scopes.Contains(thisScope))
                    {
                        issue.Scopes.Attach(thisScope); //the scope already exists
                    }
                    else
                    {
                        issue.Scopes.Add(thisScope); // the scope needs to be added
                    }
                }

                if (ModelState.IsValid)
                {
                    //if (edmx.SaveChanges() != 1) throw new Exception("Unknown error. Please try again.");
                    edmx.SaveChanges();
                    TempData["message"] = string.Format("Issue #{0} successfully modified.", id);
                }
            }

But, unfortunately, that just throws the following exception:

An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.

What am I doing wrong?

Answer 1

Stubs are generally only effective for 1-* relationships.
*-* relationships introduce a different set of challenges.

Namely that when you attach both ends – unlike 1-* – you still have no idea if the relationship already exists or not.

So that means that this code:

if (issue.Scopes.Contains(thisScope))

Is probably going to return false every time.

What I would do is this:

edmx.Issues.Attach(issue);
UpdateModel(issue);
// or ctx.LoadProperty(issue, "Scopes") if it is a POCO class.
issue.Scopes.Load(); // hit the database to load the current state.

Now you need to find out what you need to add & remove from issue.Scopes.
You can do this by comparing based on ID.

i.e. if you have a set of Scope IDs you want to have related to the issue (relatedScopes)

Then this code works out what to add and what to remove.

int[] toAdd = relatedScopes.Except(issue.Scopes.Select(s => s.ID)).ToArray();
int[] toRemove = issue.Scopes.Select(s => s.ID).Except(relatedScopes).ToArray();

Now for toAdd you do this:

foreach(int id in toAdd)
{
   var scope = new Scope{Id = id};
   edmx.Scopes.Attach(scope);
   issue.Scopes.Add(scope);
}

And for each scope you need to remove

foreach(int id in toRemove)
{
   issue.Scopes.Remove(issue.Scopes.Single(s => s.ID == id));
}

By now the correct relationships should be formed.

Hope this helps

Alex

Microsoft