Custom ElementType empty getSource criteria?

Question

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 = [
        '*' => [
            'label' => Craft::t('All Entries'),
            'defaultSort' => [
                'test_plugin.dateCreated',
                'desc'
            ]
        ]
    ];

    $sources[] = ['heading' => 'Assignments'];

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

    return $sources;
}

Answer 1

You can set it to ':empty:', as mentioned in the DbHelper::parseParam() documentation.