class DoCd extends DataObject {

    private static $db = array(
            'id_linea' => 'Varchar(10)',
            [... snip ...]
    );

    public function getCMSFields() {
        $fields = parent::getCMSFields();

        /* Linea */
        $field = new DropdownField('id_linea', 'Linea', DoSoftwareLinee::get()->map('ec_id', 'nome'));
        $fields->insertBefore($field, 'data');

        /* GRIDFIELD */
        $gridFieldConfig = GridFieldConfig_RelationEditor::create();
        $field = new GridField(
            'Software', 'All Software', DataObject::get('DoCdSoftware', "id_cd = {$this->ID}"), $gridFieldConfig
        );

        $fields->addFieldToTab('Root.Software', $field);

        return $fields;
    }

}