new code logic for saving tree.. should work fine even if the node positions fail

app/models/item.rb

@@ -44,17 +44,23 @@ class Item
     a
   end
 
-  def shift_to(new_parent, position)
+  def find_by_parent_and_position(parent, position)
+    parent.children.detect{|child| child.position == position}
+  end
+
+
+   def shift_to(new_parent, position, next_sibling_id)
     position = position.to_i
     new_parent = Item.find(new_parent)
-    current_position_sibling = find_by_parent_and_position(new_parent, position)
-    if current_position_sibling
-      if self.position > current_position_sibling.position
-        self.move_above(current_position_sibling)
-      else
-        self.move_below(current_position_sibling)
-      end
-    elsif self.parent != new_parent
+    
+    if next_sibling_id
+      next_sibling = Item.find(next_sibling_id)
+      self.move_above(next_sibling)
+    else
+      self.move_to_bottom
+    end
+
+    if self.parent != new_parent
       self.parent = new_parent
       save
     end
@@ -86,10 +92,6 @@ class Item
     end
   end
 
-  def find_by_parent_and_position(parent, position)
-    parent.children.detect{|child| child.position == position}
-  end
-
   # Enable the validation for parent_id
   def validates_presence_of_parent_id?
     true