class TestAdmin extends ModelAdmin {
    static $managed_models = array('MyDataObject');
    static $url_segment = 'testadmin';
    static $menu_title = 'TestAdmin';
}

class MyDataObject extends DataObject {
    private static $db = array('Name' => 'Varchar(255)');
    private static $has_many= array('AnotherDataObjects' => 'AnotherDataObject');

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

        if ($grid = $fields->dataFieldByName('AnotherDataObjects')) {
            $grid->getConfig()
                ->removeComponentsByType('GridFieldAddExistingAutocompleter')
                ->addComponent(new GridFieldOrderableRows('Priority'));

            $fields->removeByName('AnotherDataObjects');
            $fields->insertAfter($grid,'Name');
        }

        return $fields;
    }
}

class AnotherDataObject extends DataObject {
    private static $db = array(
        'Name'      => 'Varchar(255)',
        'Type'      => "Enum('Middle,Top,End','Middle')",
        'Priority'  => 'Int'
    );
    private static $has_one = array('MyDataObject' => 'MyDataObject');

function onBeforeWrite() {
    parent::onBeforeWrite();

    if($this->Type == 'Top')
        $this->Priority = DB::query("SELECT MIN(Priority) FROM Type = 'Top'")->value();

    if($this->Type == 'End')
        $this->Priority = DB::query("SELECT MAX(Priority) FROM AnotherDataObjectWHERE Type = 'End'")->value() + 1;
}
}