Using JDatabaseQuery::union() causes errors

Question

I'm getting errors when using JDatabaseQuery::union() to create a database query.

With Debug disabled I get this at the bottom of the page, presumably when trying to disconnect:

Warning: mysqli::stat(): Couldn't fetch mysqli in... /libraries/joomla/database/driver/mysqli.php on line 219

Warning: mysqli_close(): Couldn't fetch mysqli in... /libraries/joomla/database/driver/mysqli.php on line 226

With Debug enabled I get a further message when the query's being made:

Notice: unserialize(): Error at offset... in... /libraries/joomla/database/query.php on line 1472

That error refers to this line from JDatabaseQuery::__clone():

$this->{$k} = unserialize(serialize($v));

This issue is repeated across different MySQL, PHP 5.x and Joomla versions on XAMPP and Ubuntu Server. I originally discovered it while developing a component, but it shows up on a clean Joomla install with a clean database on a clean server. Caching is disabled, session times at 120mins. To repeat the issue, replace ContentModelArticles::getListQuery() in /administrator/components/com_content/models/articles.php with:

protected function getListQuery()
    {
        // Create a new query object.
        $db = $this->getDbo();
        $query = $db->getQuery(true);
        $sub = $db->getQuery(true);

        // Select the required fields from the first table.
        $query->select('id')
            ->from('#__content');

        // Select the required fields from the second table.
        $sub->select('content_id')
            ->from('#__content_rating');

        $query->union($sub);

        return $query;
    }

If I make the subquery a string rather than a JDatabaseQueryMysqli object, however, there are no problems:

protected function getListQuery()
    {
        // Create a new query object.
        $db = $this->getDbo();
        $query = $db->getQuery(true);

        // Select the required fields from the first table.
        $query->select('id')
            ->from('#__content');

        $query->union('SELECT content_id FROM #__content_rating');

        return $query;
    }

If I'm doing anything wrong, it's with the query objects. I've had a look at the relevant docs, found this forum post, and followed the advice to copy the example from /administrator/components/com_finder/helpers/indexer/query.php, which clones and modifies the first query object rather than creating a new one. All to no avail. There's some indication in that forum thread and in other places online that this may be a bug, but everything I've found is fairly old and that bug was apparently addressed.

Is this an issue with Joomla or me?

Answer 1

I've had some weird experiences with Jdatabase and union queries. Sometimes, if I can't manage to get the syntax correct for that, what I do is just something like $query="[write your query manually here]" and then $db->setQuery($query), etc. Probably not much help, but figured I'd add that...