code: public function generateAjax($id, $strField, $level)
  	{
  		if (!Environment::get('isAjaxRequest'))
  		{
  			return '';
  		}
  
  		$this->strField = $strField;
  		$this->loadDataContainer($this->strTable);
  
  		// Load current values
  		switch ($GLOBALS['TL_DCA'][$this->strTable]['config']['dataContainer'])
  		{
  			case 'File':
  				if (Config::get($this->strField) != '')
  				{
  					$this->varValue = Config::get($this->strField);
  				}
  				break;
  
  			case 'Table':
  				if (!$this->Database->fieldExists($this->strField, $this->strTable))
  				{
  					break;
  				}
  
  				$objField = $this->Database->prepare("SELECT " . Database::quoteIdentifier($this->strField) . " FROM " . $this->strTable . " WHERE id=?")
  										   ->limit(1)
  										   ->execute($this->strId);
  
  				if ($objField->numRows)
  				{
  					$this->varValue = StringUtil::deserialize($objField->{$this->strField});
  				}
  				break;
  		}
  
  		$this->getPathNodes();
  
  		// Load the requested nodes
  		$tree = '';
  		$level *= 20;
  
  		$objPage = $this->Database->prepare("SELECT id FROM tl_page WHERE pid=? ORDER BY sorting")
  								  ->execute($id);
  
  		while ($objPage->next())
  		{
  			$tree .= $this->renderPagetree($objPage->id, $level);
  		}
  
  		return $tree;
  	}
docs: Generate a particular subpart of the page tree and return it as HTML string
  
  @param integer $id
  @param string  $strField
  @param integer $level
  
  @return string