I was looking a while for the answer to this but couldn't find it.

I have a DataObject "Testimonial" that is a part of a Page class "BookPage", and I've also added testimonial as a ModelAdmin to list all the testimonials and the books they are under. I used the ModelAdmin to switch Books that the testimonial belongs in, with a "BookPage" dropdown....

	public function getCMSFields(){
		$fields = new FieldList();
		...
		$fields->push( new DropdownField(
		    'BookPageID',
		    'Please choose a Book',
		    $this->getBookSelection() ) );

...
	function getBookSelection()
	{
	    if($Pages = DataObject::get('BookPage'))
	    {
	    	return $Pages->map('ID', 'Title', 'Please Select');
	    }
	    else
	    {
	    	return array('No Objects found');
	    }
	} 

I'd like to be able to show the dropdown menu only when I'm adding or editing a testimonial from the modelAdmin (/admin/testimonial/Testimonial/EditForm/field/Testimonial/item/2/edit) ,
but not when I'm accessing the testimonial from editing the pages (/admin/pages/edit/EditForm/field/Testimonials/item/new).