containable.php

<?php

class ArticlesController extends AppController
{
    public $components = array('Paginator');
    public function test()
    {
        $this->Paginator->settings['Article'] = array(
            'contain' => array('Category' => array(
                'conditions' => array('Category.id' => 2)
            )),
        );

        $articles = $this->Paginator->paginate('Article');

        var_dump($articles); die();
    }
} 

Output