i'm using odoo11 and i have installed a custom module to calculate the overtime. i have many team leaders. Every team leader he has a number of employee .I want to limit the approve action for the manager in a way every manager approve request of his own employees only. i have override the approve action to verify if the person who's gone approve is his manager or not but always it allows any manager to approve all the requests. Here is code :

overtime.py

class BtHrOvertime(models.Model):
_name = "bt.hr.overtime"
_description = "Bt Hr Overtime"
_rec_name = 'employee_id'
_order = 'id desc'

employee_id = fields.Many2one('hr.employee', string="Employee")
manager_id = fields.Many2one('hr.employee', string='Manager')
start_date = fields.Datetime('Date check in')
end_date = fields.Datetime('Date check out')
overtime_hours = fields.Float('Overtime Hours')
notes = fields.Text(string='Notes')
state = fields.Selection([('draft', 'Draft'), ('confirm', 'Waiting for Approval'), ('refuse', 'Refused'),
                          ('validate', 'Approved'), ('cancel', 'Cancelled')], default='draft', copy=False)
attendance_id = fields.Many2one('hr.attendance', string='Attendance')
user_id = fields.Many2one('res.users', string='User', related='employee_id.user_id', related_sudo=True,
                          compute_sudo=True, store=True, default=lambda self: self.env.uid, readonly=True)
parent_id = fields.Many2one('bt.hr.overtime', string='Parent', copy=False)

@api.multi
def action_approve(self):
    if (self.employee_id.parent_id.user_id.id != uid):
        raise exceptions.UserError(_('You cannot approve.'))
    elif  self.attendance_id.check_in and self.attendance_id.check_out:
            start_date = datetime.datetime.strptime(self.attendance_id.check_in, DEFAULT_SERVER_DATETIME_FORMAT)
            end_date = datetime.datetime.strptime(self.attendance_id.check_out, DEFAULT_SERVER_DATETIME_FORMAT)
            difference = end_date - start_date
            hour_diff = str(difference).split(':')[0]
            min_diff = str(difference).split(':')[1]
            tot_diff = hour_diff + '.' + min_diff
            actual_working_hours = float(tot_diff)
            for record in self:
                if actual_working_hours > 9:
                    record.overtime_hours = actual_working_hours - 9
                    record.start_date = record.attendance_id.check_in
                    record.end_date = record.attendance_id.check_out
                else:
                    record.overtime_hours = 0
                    record.start_date = record.attendance_id.check_in
                    record.end_date = record.attendance_id.check_out
    return self.write({'state': 'validate'})