how to query posts by category and tag?

Edit: See below for proper way to query category and tag intersections.

global $wp_query;
        $args = array(
        'category__and' => 'category', //must use category id for this field
        'tag__in' => 'post_tag', //must use tag id for this field
        'posts_per_page' => -1); //get all posts

$posts = get_posts($args);
        foreach ($posts as $post) :
  //do stuff 
     endforeach;