Getting data from migxdb in json format how?

Question

MODX

How can I get data from migxdb table in json format?

$output = $modx->runSnippet('migxLoopCollection', array(
   'package' => 'rooms',
   'classname' => 'room'
));

$jsonString = $modx->toJSON($output);

echo $jsonString;

it doesn't work that way