[SOLVED] Fighting again with only_full_group_by!

Need Help
12

only full group by is the default setting for MySQL 5.7 and above

In your case the easiest soulution is to call select() and group() methods on the query with the same filed list.
So you have to end up with something like this.

$discoveryBooks = $this->DiscoveryBooks->find()
  ->select(['DiscoveryBooks.id', 'DiscoveryBooks.name', 'DiscoveryCards.name'])
  ->matching('DiscoveryCards', function ($q) use ($siteIds){
            return $q->where([
                'DiscoveryCards.site_id IN' => $siteIds
            ]);
        })
  ->group(['DiscoveryBooks.id', 'DiscoveryBooks.name', 'DiscoveryCards.name'])
  ->where([
            'published' => true,
            'approved' => true
        ])
  ->all();