Custom ElementType empty getSource criteria? I'm trying to add a source on a custom element type that shows entries with an empty assigned_user_id field. This criteria is not honored and returns the same results as the "All Entries" source. Is there any way to specify a null or empty string criteria value?

public function getSources($context = null)
{
    $sources = [
        '*' =&gt; [
            'label' =&gt; Craft::t('All Entries'),
            'defaultSort' =&gt; [
                'test_plugin.dateCreated',
                'desc'
            ]
        ]
    ];

    $sources[] = ['heading' =&gt; 'Assignments'];

    $sources['unassigned'] = [
        'label'    =&gt; Craft::t('Unassigned'),
        'criteria' =&gt; ['assigned_user_id' =&gt; '']
    ];

    return $sources;
}
You can set it to ':empty:', as mentioned in the DbHelper::parseParam() documentation.