Update only scalar properties in Code First, EF 5 - entity-framework

Question

God day!
I have a tree of entities and at specific point of time i need to update only scalar properties of one entity. Classic update rise entire graph lookup, but relations not need to update.
The trouble in Category entity what one category have another categories in children. My method generate exceptions when saving changes about duplicate key. I think EF try to add children to database.
Static method of my data context listed below:
public static void Update<T>(T item) where T : KeyedObject
{
if (item == null)
throw new ArgumentNullException("Item to update is null");
item.ValidateIsNotNew();
using (DataContext db = new DataContext())
{
T original = GetOriginalWithException<T>(db, item);
DbEntityEntry entry = db.Entry(original);
entry.CurrentValues.SetValues(item);
entry.State = EntityState.Modified;
try
{
db.SaveChanges();
}
catch (Exception ex)
{
throw new DatabaseException(
"Cant update list item. See inner exception for details.",
ex);
}
}
}
I tries another method: attaching object. This method does not throw exception, but it rise entire graph update and take many resources. Code listed below:
public static void Update<T>(T item) where T : KeyedObject
{
if (item == null)
throw new ArgumentNullException("Item to update is null");
item.ValidateIsNotNew();
using (DataContext db = new DataContext())
{
db.Set<T>().Attach(item);
db.Entry(item).State = EntityState.Modified;
try
{
db.SaveChanges();
}
catch (Exception ex)
{
throw new DatabaseException(
"Cant update list item. See inner exception for details.",
ex);
}
}
}