[IMP]  "X in self" is now equivalent to any(X == rec for rec in self)

The former behavior was confusing: ``X in self`` was equivalent to ``X <= self`` (set inclusion).
In particular, a condition like::

    self.instructor_id in self.attendee_ids

evaluates to True when self.instructor_id is empty, which is not what you would expect intuitively from the condition.