I was wondering if its possible to set the ParentID of a pagetype to specific page. The pagetype is being controlled by a ModelAdmin instead of the pages interface. I've tryed setting the $defaults array in the __construct() but I get a class does not exist error

       static $defaults;	



	/**
	 * Static methods
	 * ----------------------------------*/
	 
	public function __construct() {
		
	
		$mainListingPage = SiteTree::get_by_link("listings");
		
		
		$this->defaults = array(
			"ParentID" => $mainListingPage->ID,
		);
		
		parent::__construct();
	}

Should I just handle it in the onBeforeWrite ?

Use onBeforeWrite().

Calling it in onBefore write was failing I ended up adding a hidden ParentID field to the form and setting the value with the function. When I have a chance I'll try onBeforeWrite again so I can trackdown the actual issue.