public $fields = array(
        'id' => array('type' => 'string', 'null' => false, 'default' => null, 'length' => 36, 'collate' => 'utf8_general_ci', 'charset' => 'utf8', 'key' => 'primary'),
        'blog_id' => array('type' => 'string', 'null' => false, 'default' => null, 'length' => 36, 'collate' => 'utf8_general_ci', 'charset' => 'utf8'),
        'tag_id' => array('type' => 'string', 'null' => false, 'default' => null, 'length' => 36, 'collate' => 'utf8_general_ci', 'charset' => 'utf8'),
        'indexes' => array(