odoo/odoo — pull request #2127 by rco-odoo (2014-09-02 09:16:18+00:00)
Title: [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.