protected function makeMutatableFields(Collection $fields = null)
    {
        if (is_null($fields))
        {
            return '';
        }

        $mutatables = [];

        foreach ($fields as $field)
        {
            if (in_array($field->type, ['document', 'gallery', 'markdown', 'node', 'node_collection']))
            {
                $mutatables[] = "'{$field->name}' => '{$field->type}'";
            }
        }

        return implode(",", $mutatables);
    }