Constrain and Unique field

    _sql_constraints = [('issue_date_project_uniq', 'unique (issue_date, project)',
                         "Combination of project and date already exists !")]

    @api.constrains('issue_date', 'project')
    def _check_project_unique(self):
        record_count = self.search_count([('issue_date', '=', self.issue_date),
                                           ('project', '=', self.project.id),
                                           ('id', '!=', self.id)])
        if record_count > 0:
            raise ValidationError("Record already exists!")