Updating a entity using Entity Framework 4.3 - Code First

Question

I'm having a hard time updating entities in Entity Framework.

The scenario: - I load a entity with new DbContext().GetById(guid) - I try to save this entity using a extension method, then using a new DbContext()

Heres my Update method:

 public virtual void Update(IEntity entityToUpdate)
    {
        var dbEntry = Context.Entry(entityToUpdate);
        if (dbEntry == null) return;

        if (Context.Entry(entityToUpdate).State == EntityState.Detached)
            DbSet.Attach(entityToUpdate);
        else
        {
            dbEntry.CurrentValues.SetValues(entityToUpdate);
            Context.Entry(entityToUpdate).State = EntityState.Modified;
        }

        Context.SaveChanges();
    }

This is a collection of attemps by me. If I use SetValues I'm told that the entity is detached and therefor not possible, and If I use the attach I get the following error: 'An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key.'

I'm obviously doing something fundamentally wrong. Can someone please help me in the right direction?

UPDATE:

    protected void TransferClubs(object sender, EventArgs e)
    {
        var clubHelper = new ClubHelper();
        var club = clubHelper.GetClub(new Guid("A009D0CD-71C4-42E8-88E2-037F059B12EE"));
        club.AddUser(Guid.NewGuid(), ClubRoleType.Admin);
        club.AddUser(Guid.NewGuid(), ClubRoleType.Admin);

        club.Save();
    }

    public static bool Save(this ClubItem item)
    {
        var clubHelper = new ClubHelper();
        clubHelper.AddOrUpdate(item);
        return true;
    }

    public ClubItem AddOrUpdate(ClubItem item)
    {
        if (item.Id == Guid.Empty)
            Insert(item);
        else
            Update(item);

        return item;
    }

And the Update() method you see in my original post...

Can you give an example of the full context between loading the entity, changing it and then calling this Update method? You say that Update happens with a "new DbContext()" but then you have an exception about already existing (=attached) objects with the same key. I can't imagine how this could happen in a new context. — Slauma
– Slauma, Commented Jun 14, 2012 at 11:10
Added all the methods in play. The AddUser adds new Users to a ICollection<ClubUsers> collection — Kulvis
– Kulvis, Commented Jun 14, 2012 at 11:30
ClubHelper creates a new context, GetClub loads an existing club from the DB and AddUser is supposed to create a new user, insert it into the DB and set a reference to the club, right? — Slauma
– Slauma, Commented Jun 14, 2012 at 12:00
thats correct. public ICollection<ClubRoleItem> ClubUsers { get; set; } is the UserCollection on the Club. — Kulvis
– Kulvis, Commented Jun 14, 2012 at 13:02

Slauma · Accepted Answer · 2012-06-14 14:29:10Z

2

In my opinion the update should work if you replace in TransferClubs the last line club.Save(); by

clubHelper.SaveChanges();

This method should simply call Context.SaveChanges(); and save the changes to the loaded entity, i.e. create two new users with a foreign key set to the loaded club.

Honestly your approach to update an entity is rather strange and complicated. I have no idea why your code throws an "An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key" exception. But there are several flaws and things which don't make sense:

You are not disposing your instantiated contexts. If ClubHelper creates a new context it should dispose it when it goes out of scope. So, ClubHelper should implement IDisposable and the Dispose inplementation should call context.Dispose(). Then you can use a using that disposes the instantiated object automatically at the end:
```
protected void TransferClubs(object sender, EventArgs e)
{
    using (var clubHelper = new ClubHelper())
    {
        // stuff...
    } // Dispose called here automatically
}
```
These lines don't make sense:
```
var dbEntry = Context.Entry(entityToUpdate);
if (dbEntry == null) return;
```
If entityToUpdate is an entity of your object model dbEntry is never null. It only could have the state Detached. If entityToUpdate is not an entity of your object model Entry throws an exception but doesn't return null.
The if case doesn't make sense:
```
if (Context.Entry(entityToUpdate).State == EntityState.Detached)
    DbSet.Attach(entityToUpdate);
// ...
Context.SaveChanges();
```
Attach adds an entity to the context in state Unchanged. If you just call SaveChanges after that nothing happens and will be written to the database because nothing has changed.
Also the else case doesn't make sense:
```
dbEntry.CurrentValues.SetValues(entityToUpdate);
Context.Entry(entityToUpdate).State = EntityState.Modified;
```
SetValues copies the properties of entityToUpdate to the properties of the entity which has the same key and is already attached to the context if the properties have the same names. If the property values are different it marks the property as Modified. If you set the whole entity afterwards to Modified you mark all properties as Modified which makes SetValues before redundant.

In addition both lines won't help you to create the needed UPDATE statements because they don't mark a relationship as modified and affect only scalar (and complex) properties but not navigation properties. But updating relationships - namely between club and two new users - is exactly what you need in your example.
Finally it doesn't make sense to perform the update in a completely new context...
```
var clubHelper = new ClubHelper();
clubHelper.AddOrUpdate(item);
```
... when you have loaded and changed the entity just in the lines before in another context. Entity Framework has already done all the work to track your changes in the first context to generate the needed SQL statements. If you create a new context after this where you want to save the changes in you throw all the work away and have to start from scratch to tell EF what changes you have done to the original entity.

edited Jun 14, 2012 at 14:29

answered Jun 14, 2012 at 14:10

Slauma

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4 Comments

Kulvis Over a year ago

The code pasted by me was gathered from different places in the solution and did not present you with all of my needs, sorry for that. There are places where the same clubhelper isn't available and I then need the extension method. The Update method was, as I mentioned in my post, a collection of some of my attemps to grasp the issue :) Should have cleaned it up before pasting. I'm still having some issues with this but I try to change my usage of the clubHelper and see if I can simplify this a bit. Thanks a lot for a very detailed and clarifying answer :)

Slauma Over a year ago

@Kulvis: I misunderstood it a bit then. I'm critizing code which doesn't really exist in that form, sorry! Hopefully the answer is useful at all...

Kulvis Over a year ago

You were correct in critizing the code and we have now tried to make it more simple. We still need the extension-method but we are not getting the "..key exists..." error anymore. The new issues is that noe entities in the ClubUsers collection is saved to the database. After attaching the clubitem they are not detected as new by the context. I change the state of the clubitem to modified, but do I also have to do this on each of the child ClubUser-object?

Slauma Over a year ago

@Kulvis: Yes, if the ClubUser objects are existing entities. If they are new you have to put them into Added state though.

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