Entity Framework linked list - entity-framework

Question

I have an entity (I am using code first) that looks like that:
public class Node
{
public int ID { get; set; }
public string SomeInfo { get; set; }
public virtual Node Previous { get; set; }
public virtual Node Next { get; set; }
}
There is no problem to save the Next Node for example. However if the ID of Previous is 1 and I try to set the Next Node (wich is the one with ID=1) to 2 this exception is thrown.
The object cannot be added to the object context. The object�s
EntityKey has an ObjectStateEntry that indicates that the object is
already participating in a different relationship.
I am saving the node like this:
int nextId;
int previousId;
if (int.TryParse(Request["previous"], out previousId))
node.Previous = this.nodeRepository.GetSingle(previousId);
if (int.TryParse(Request["next"], out nextId))
node.Next = this.nodeRepository.GetSingle(nextId);
this.nodeRepository.Update(node);
Update looks like this:
public virtual void Update(T entity)
{
this.context.Entry(GetSingle(entity.ID)).State = EntityState.Detached;
this.context.Entry(entity).State = EntityState.Added;
this.context.Entry(entity).State = EntityState.Modified;
this.Save();
}
And GetSingle like this:
public virtual T GetSingle(object id)
{
var query = this.entities.Find(id);
return query;
}
UPDATE 1
The line with the exception is in the Update method:
this.context.Entry(entity).State = EntityState.Modified;

Answer 1

Is your context in a Using block and being disposed of at some point? I've had similar 'The object cannot be added to the object context." errors. I would add Id's for Previous and Next to your model and use those to update the foreign keys.
public class Node
{
public int ID { get; set; }
public string SomeInfo { get; set; }
public virtual Node Previous { get; set; }
public int PreviousId { get; set; }
public virtual Node Next { get; set; }
public int NextId { get; set; }
}
To update the foreign keys...
int nodeId; // I'm assuming you know the id of node you want updated.
int nextId;
int previousId;
using (var context = new Context())
{
// Perform data access using the context
var node = context.Nodes.find(nodeId);
node.NextId = nextId;
node.PreviousId = previousId;
context.SaveChanges();
}