gistfile1.php

<?php

$q = $modx->newQuery('modResource');
$q->innerJoin('ShopmodxProduct', 'Product');
$q->leftJoin('modTemplateVarResource', 'tv', 'tv.contentid=modResource.id and tv.tmplvarid=29');
$q->where(array(
    "tv.id" => null,
));

print $modx->getCount('modResource', $q);

foreach($modx->getCollection('modResource', $q) as $doc){
    $doc->setTVValue(29, 1);
}