I'd like to know if CakePHP3(.1) is supposed to manage multiselect values by itself when we edit an object? I mean, check already selected options and affect modifications like for simple datas. According to my tests (but maybe I didn't initialised it correctly) and Cakephp 3 Unable to mark multiple checkboxes selected, it seems not to be the case.
<?= $this->Form->create($site, ['type' => 'file', 'horizontal' => true])?>
<?= $this->Form->input('sitecategories', ['options' => $categories, 'label' => __("Catégories :"), 'placeholder' => __("Catégorie du site"), 'multiple' => true]); ?>
....
$site contains an array of sitecategories.