Restrict number of topics & page list

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Good day,

Is it possible to restrict how many topics a user can select for a page? I'm working on a project where ideally the user would only be able to select one topic per page.

My second questions relates to this. In a page list, I want to set additional CSS classes on HTML elements (the grid-items) depending on the selected topic. I've modified the thumbnail_grid template as below, but I'm not sure whether this is the best way to achieve this.

<?php foreach ($pages as $page):
$topics = $page->getAttribute('category_topic');
$topicClass;
if ($topics) {
    foreach ($topics as $topic) {
        // get the tree node ID
        $treeNodeID = $topic->getTreeNodeID();
        // get the tree node name
        $treeNodeName = $topic->getTreeNodeDisplayName();
        switch ($treeNodeName) {
            case 'Seminars':
            $topicClass='topic-seminars';
            break;
            case 'Events':
            $topicClass='topic-events';


What do you think?

Cheers,
B.