def add_spectator(self, user):
"""Add a spectator to the game.
:param user: User -- The spectator's `User` object.
:raise: ValueError if `Game` does not allow spectators, or spectator
slots have been filled.
"""
if self.options.spec_mode is Game.SpecMode.NOSPEC:
raise ValueError("This game does not allow spectators.")
if len(self.spectators) >= self.options.spec_max:
raise ValueError("This game has reached the max allowed "
"spectators.")
self.spectators.append(Spectator(user))
self._update_model_collection('spectators', {'action': 'append'})
