private function prepareFields(ApiContentType $contentType, $fields = null)
    {
        if ($fields !== null) {
            if (strpos($fields, ',') !== false) {
                return explode(',', $fields);
            }

            return array($fields);
        }

        $fields = array();
        $contentFields = $contentType->getFieldDefinitions();

        foreach ($contentFields as $field) {
            $fields[] = $field->identifier;
        }

        return $fields;
    }