Masterpiece, highly optimised :)

<?php
//TODO: Add PHPDoc
//By Darko Lukic
class Category extends CActiveRecord
{
    const TYPE_MAIN = 1;
    const TYPE_CATEGORY = 2;
    const TYPE_TEST = 3;
    const TYPE_CUSTOM_TEST = 4;

    const VISIBILITY_HIDDEN = 0;
    const VISIBILITY_VISIBLE = 1;
    const VISIBILITY_HIGHLIGHTED = 2;

    private $parents;
    private $childs;

    public static function model($className = __CLASS__) {
        return parent::model($className);
    }

    public function tableName() {
        return '{{categories}}';
    }

    public function rules() {
        return array(
            'parent_category' => array(self::BELONGS_TO, 'Category', 'category_id'),
            'child_categories' => array(self::HAS_MANY, 'Category', 'category_id'),
            'questions' => array(self::HAS_MANY, 'Question', 'category_id'),
        );
    }


    public function attributeLabels() {
        return array(

        );
    }


    public function getParents()
    {
        if ($this->parents != NULL)
            return $this->parents;


        $this->parents = array();
        $parentModel = $this->getParent();

        if ($parentModel)
            $this->parents[] = $parentModel;
        else return false;


        while($parentModel)
        {
            $parentModel = $this->getParent($parentModel->category_id);

            if ($parentModel)
                $this->parents[] = $parentModel;
        }

        return array_reverse($this->parents);
    }

    public function getParent($category_id = NULL)
    {
        if ($category_id == NULL)
        {
            if (isset($this->parents[0]))
                return $this->parents[0];

            return self::model()->findByAttributes(array('id' => $this->category_id));
        }

        if ($this->parents != NULL)
            foreach($this->parents as $parent)
                if ($parent->id == $category_id)
                    return $parent;

        return self::model()->findByAttributes(array('id' => $category_id));
    }

    public static function getParentById($category_id)
    {
        return self::model()->findByAttributes(array('id' => $category_id));
    }


    public function getChilds()
    {
        if ($this->childs != NULL)
            return $this->childs;

        $this->childs = self::model()->findAllByAttributes(array('category_id' => $this->id));
        return $this->childs;
    }

    public function getTimeLimit()
    {
        if (empty($this->time_limit))
        {
            $parentModel = $this->getParent();

            if (!$parentModel)
                return 0;

            while (empty($parentModel->time_limit) && $parentModel->type != self::TYPE_MAIN)
                $parentModel = $this->getParent();

            return $parentModel->time_limit;
        }

        return $this->time_limit;
    }

    public function getImageUrl()
    {
        $imagesPath = Yii::app()->basePath.DIRECTORY_SEPARATOR.'..'.DIRECTORY_SEPARATOR.'images'.DIRECTORY_SEPARATOR.'categories'.DIRECTORY_SEPARATOR;

        if (!file_exists($imagesPath.$this->id.'.png'))
        {
            $parentModel = $this->getParent();
            while (!file_exists($imagesPath.$parentModel->id.'.png'))
                $parentModel = $this->getParent();

            return Yii::app()->createUrl('images/categories/'.$parentModel->id.'.png');
        }

        return Yii::app()->createUrl('images/categories/'.$this->id.'.png');
    }

    public static function createBreadcrumbs($categoryParents, $category)
    {
        $breadcrumbs = array();

        if ($categoryParents)
            foreach ($categoryParents as $categoryParent)
                $breadcrumbs[$categoryParent->title] = Yii::app()->createUrl('category/view/', array('id' => $categoryParent->id));

        $breadcrumbs[] = $category->title;

        return $breadcrumbs;
    }

    public static function isTest($type)
    {
        if ($type == self::TYPE_TEST || $type == self::TYPE_CUSTOM_TEST)
            return true;
        else return false;
    }
}