votes_table = Table('votes', metadata,
	Column('user_id', Integer, ForeignKey('users.id'), nullable=False),
	Column('post_id', Integer, ForeignKey('posts.id'), nullable=True),
	Column('comment_id', Integer, ForeignKey('comments.id'), nullable=True),
	Column('timestamp', DateTime(), nullable=False, default=func.current_timestamp()),
	Column('value', Integer, nullable=False, default=0)
	)

Vote.mapper = Session.mapper(Vote, votes_table,
	primary_key = [ votes_table.c.user_id, votes_table.c.post_id, votes_table.c.comment_id ],
	properties = {
		'user': relation(User),
		'comment': relation(Comment),
		'post': relation(Post)
	}
	)