You can make a fields as function type

in odoo-8.0 there is the compute attribute to make the field as a function

For Example: seats_reserved = fields.Integer(oldname='register_current', string='Reserved Seats', store=True, readonly=True, compute='_compute_seats')

@api.multi
@api.depends('seats_max', 'registration_ids.state', 'registration_ids.nb_register')
def _compute_seats(self):
    """ Determine reserved, available, reserved but unconfirmed and used seats. """
    # initialize fields to 0
    for event in self:
        event.seats_unconfirmed = event.seats_reserved = event.seats_used = 0
    # aggregate registrations by event and by state
    if self.ids:
        state_field = {
            'draft': 'seats_unconfirmed',
            'open':'seats_reserved',
            'done': 'seats_used',
        }
        query = """ SELECT event_id, state, sum(nb_register)
                    FROM event_registration
                    WHERE event_id IN %s AND state IN ('draft', 'open', 'done')
                    GROUP BY event_id, state
                """
        self._cr.execute(query, (tuple(self.ids),))
        for event_id, state, num in self._cr.fetchall():
            event = self.browse(event_id)
            event[state_field[state]] += num
    # compute seats_available
    for event in self:
        event.seats_available = \
            event.seats_max - (event.seats_reserved + event.seats_used) \
            if event.seats_max > 0 else 0

You can see in the event module in event.py file

Thanks