Dependencies By Expression While Caching

I have fetched model. I need to cache view using this model and refresh cache when record updated (property updated_at).

        $dependency = new CExpressionDependency("'" . $content->updated_at . "'");

        if ($this->beginCache($cacheId, array(

                    'duration' => 24 * 3600,

                    'dependency' => $dependency,

                ))) {

            echo mktime();



$cacheId is unchanged.

At first run cache file is generated.

When I update model the cache is not regenerated. I also tried just to change expression

        $dependency = new CExpressionDependency("'xyz" . $content->updated_at . "'");

but it doesn’t help either.

What should I configure more?