How to set a custom parent link in child resource.

self link of my onFetch action is:

http://localhost/project/category/12

I want to add link to onFetchAll action in current Halresource like this:
_links: {
category_link: {
href: "http://localhost/project/content/categories"
}

My code is:

$cats = new Link('category_link');
$cats->setRoute('content/categories', array(), array());
$links->add($cats,true);

but it always result this:

links: {
self: {
href: "http://localhost/project/content/categories/12"
}-
category_link: {
href: "http://localhost/project/content/categories/12"
}

How can i have what i want ?