def register_to_parent(item)
      fail Error, 'None belonging' unless parent_id?(item)
      ids << item.id
      self << item
      item
    end
def register_to_parent(item)
      fail Error, 'None belonging' unless parent_id?(item)
      ids << item.id
      self << item
      item
    end
Register a resource in collection

 @param item [BlockScore::Base] a resource

 @raise [BlockScore::Error] if no `parent_id`
 @return [BlockScore::Base] otherwise

 @api private