Workaround: button in tree view with following function in model (assuming a1 and a2 are one2many fields):

@api.multi
    def go_view(self):
        self.ensure_one()
        record = self.a1 if self.a1 else self.a2 if self.a2 else False
        return {
            'type': 'ir.actions.act_window',
            'res_model': record._name,
            'view_mode': 'form',
            'view_id': self.env.ref('test.view_' + record._name + '_form').id,
            'res_id': record.id,
        }