Many_many

class PostPage extends Page {
  public static $many_many = array(
    'Authors' => 'AuthorPage'
  );
}

class AuthorPage extends Page {
  public static $belongs_many_many = array(
    'Posts' => 'PostPage'
  );
}

class HomePage_Controller extends Page_Controller {

	public function filterStuff() {
		$results = PostPage::get()->filter(array('Authors.ID:ExactMatch' => 37));
		return $results->renderWith('SomeTemplate');
	}
}