code: def add_file(self, file_to_add):
          
          from .files import File
          assert isinstance(file_to_add, File), "Files being added must be instances of a subclass of File class"
          file_to_add.node = self
          if file_to_add not in self.files:
              self.files.append(file_to_add)
docs: add_file: Add to node's associated files
              Args: file_to_add (File): file model to add to node
              Returns: None