def add_child(node)
      # bulk update nodes by a sql query
      _update_nodes(r_ptr, r_ptr, "r_ptr >= #{r_ptr}")

      # child node's pointer
      node.l_ptr = r_ptr
      node.r_ptr = r_ptr + 1
      node.depth = depth + 1
      node.save
    end
def add_child(node)
      # bulk update nodes by a sql query
      _update_nodes(r_ptr, r_ptr, "r_ptr >= #{r_ptr}")

      # child node's pointer
      node.l_ptr = r_ptr
      node.r_ptr = r_ptr + 1
      node.depth = depth + 1
      node.save
    end
class methods
 =========================================================
    instance methods
 =================
 Instance Methods
 =================