Creating an Update method in a Repository - attaching/detaching entities not working - asp.net-mvc

Question

I have the following Update method:
public Folder UpdateFolder(Folder folder)
{
_db.Folders.Attach(folder); // error happens here
var entry = _db.Entry(folder);
entry.Property(e => e.Title).IsModified = true;
SaveChanges();
return entry.Entity;
}
I get "An object with the same key already exists" when I try to Attach. If I remove that line, I get "The entity of type "folder" does not exist in this context".
I then try the following:
public Folder UpdateFolder(Folder folder)
{
var entry = _db.Entry(folder);
entry.State= EntityState.Detached;
_db.Folders.Attach(folder);
entry.Property(e => e.Title).IsModified = true;
And I get the same error when I hit the call to Attach.
Here's where I'm calling it from (test method):
// Update Folder, Check Folder
homeFolder.Title = "Updated";
_dtoServices.UpdateFolder(homeFolder); // HERE
Assert.AreEqual(_dtoServices.GetFolder(homeFolder.FolderId).Details, "Updated");
In my DtoServices:
public FolderDto UpdateFolder(FolderDto folderDto)
{
var test = _repository.UpdateFolder(folderDto.ToEntity());
return null;
}
In my FolderDto:
public class FolderDto
{
public FolderDto()
{
}
public FolderDto(Folder folder)
{
FolderId = folder.FolderId;
Title = folder.Title;
}
[Key]
public int FolderId { get; set; }
[Required]
public string Title { get; set; }
public Folder ToEntity()
{
var folder = new Folder
{
FolderId = FolderId,
Title = Title,
};
return folder;
}
}
Any idea why this happening? How can I build a more robust check for my update method? I've searched around but can't find anything conclusive.

Answer 1

You have to make sure the entity isn't already attached. I do something like this (TEntity : class):
...
//See if entity is attached already and return if so.
TEntity attached = GetAttached(entityToUpdate);
if(attached == null)
{
//Entity is not attached, attach it.
}
...
//Generic helper to check for attached entity
private TEntity GetAttached(TEntity aEntity)
{
var lObjectContext = ((IObjectContextAdapter)context).ObjectContext;
EntityKey lKey = GetEntityKey(aEntity);
ObjectStateEntry entry;
if (!lObjectContext.ObjectStateManager.TryGetObjectStateEntry(lKey, out entry)) return null;
if (entry.State == EntityState.Detached) return null;
return (TEntity)entry.Entity;
}