I have a nested Dataobjects to create Accordion and Accordion Tabs
I created a Accordion first and saved it and is working fine.
Now I add a new Accordion Tab to the Accordion it save without the AccordionPageID.

Accordion Page

class AccordionPage extends Page
{

	private static $has_many = array(
		"Accordions" => "Accordion",
		"AccordionTabs" => "AccordionTab",
	);
	private static $has_one = array(
		'RootFolder' => 'Folder'
	);

public function getCMSFields()
	{

		$fields = parent::getCMSFields();	

		//$fields->removeFieldFromTab('Root.Main', 'Content');

		$fields->addFieldToTab('Root', $accordion = new Tab('Accordions'));
		$accordion->setTitle(_t('AccordionPage.ACCORDION', 'Accodrions'));
		if ($rootFolder = $this->RootFolder()) {
			$accordionConfig = GridFieldConfig_RecordEditor::create();

			if(class_exists('GridFieldBulkManager')) {
				$accordionConfig->addComponent(new GridFieldBulkManager());
			}

			if(class_exists('GridFieldSortableRows')) {
				$accordionConfig->addComponent(new GridFieldSortableRows('SortOrder'));
			}
			$accordionField = new GridField('Accordions', 'Accordions', $this->Accordions(), $accordionConfig);
			$fields->addFieldToTab("Root.Accordions", $accordionField);
		} else {
			$fields->addFieldToTab(
				"Root.Accordions",
				new HeaderField(
					_t("AccordionPage.ACCORDIONNOTSAVED", "You may add accordions once you have saved the page for the first time."),
					$headingLevel = "3"
				)
			);
		}
		return $fields;
	}
}

Accordion DataObject

class Accordion extends DataObject
{

	private static $has_one = array(
		'AccordionPage' => 'AccordionPage',
		'AccordionFolder' => 'Folder'
	);

	private static $has_many = array(
		"AccordionTabs" => "AccordionTab",
	);
	function getCMSFields()
	{
		$fields = new FieldList(new TabSet('Root'));
		$fields->addFieldsToTab('Root.Main', array(
			new TextField('Name', _t('AccordionPage.ACCORDIONNAME','Accordion Name'), null, 255),
			new TextareaField('Description', _t('AccordionPage.DESCRIPTION','Description'))
		));
		if ($accordionFolder = $this->AccordionFolder()) {
			$accordionConfig = GridFieldConfig_RecordEditor::create();

			if(class_exists('GridFieldBulkManager')) {
				$accordionConfig->addComponent(new GridFieldBulkManager());
			}

			if(class_exists('GridFieldSortableRows')) {
				$accordionConfig->addComponent(new GridFieldSortableRows('SortOrder'));
			}
			$accordionField = new GridField('AccordionTabs', 'Tabs', $this->AccordionTabs(), $accordionConfig);
			$fields->addFieldToTab("Root.AccordionTabs", $accordionField);
		} else {
			$fields->addFieldToTab(
				"Root.AccordionTabs",
				new HeaderField(
					_t("AccordionPage.ACCORDIONTABNOTSAVED", "You may add tabs once you have saved the page for the first time."),
					$headingLevel = "3"
				)
			);
		}
		return $fields;
	}

AccordionTab DataObject

class AccordionTab extends DataObject
{
	private static $db = array(
		'TabName' => 'Varchar(255)',
		'Description' => 'Text',
		'SortOrder' => 'Int',
		'URLSegment' => 'Varchar(255)',
		'showContent' => 'Boolean',
		'TabContent' => 'HTMLText'
	);

	private static $has_one = array(
		'AccordionPage' => 'AccordionPage',
		'Accordion' => 'Accordion',
		'Folder' => 'Folder'
	);

	static $summary_fields = array(
		'TabName' => 'Title',
		'Description' => 'Description'
	);
	function getCMSFields()
	{
		$fields = new FieldList(new TabSet('Root'));
		$fields->addFieldsToTab('Root.Main', array(
			new TextField('TabName', _t('AccordionPage.TABNAME','Tab Title'), null, 255),
			new TextareaField('Description', _t('AccordionPage.DESCRIPTION','Description'))
		));
		return $fields;
	}

When I Add accordion Tab it does not save AccordionPage ID . it only saves the AccordionID.

I used onBeforeWrite to popuplate the PageID

	public function onBeforeWrite() {
		parent::onBeforeWrite();
		$Accordion = $this->Accordion();
		$this->AccordionPageID = $Accordion->AccordionPageID;

	}