Override mongoid-tree callback condition for default position

app/models/user/user.rb

@@ -52,6 +52,7 @@ class User
   accepts_nested_attributes_for :attribute_values, :allow_destroy => true 
 
   before_create :initialize_desktop
+  before_save :assign_default_position, :if => :assign_default_position?
   before_save :rebuild_status_record
   scope :remote_account, where(:nccu_id.ne => nil)
   scope :not_guest_user, all_of(:name.ne => "guest")
@@ -289,7 +290,7 @@ class User
     end
   end
   
-  protected
+  private
 
   def rebuild_status_record
     self.status_record = {}
@@ -299,4 +300,9 @@ class User
     end
   end
 
+  # override mongoid-tree's method
+  def assign_default_position?
+    !self['name'].eql?('guest') && (self.position.nil? || self.parent_id_changed?)
+  end  
+
 end