无法使用PHP通过Cloud Firestore从嵌套集合中获取文档

Question

在https://github.com/GoogleCloudPlatform/php-docs-samples/blob/master/firestore/src/get_all_docs.php的Google官方文档之后，我无法检索属于嵌套集合的文档，即：

$db = new FirestoreClient([
    'projectId' => $projectId,
]);
//$citiesRef = $db->collection('cities'); //working original example
$citiesRef = $db->collection('countries')->document('USA')->collection('cities'); //this throws an HTTP Error 500 
$documents = $citiesRef->documents();
foreach ($documents as $document) {
    if ($document->exists()) {
        printf('Document data for document %s:' . PHP_EOL, $document->id());
        print_r($document->fields());
        printf(PHP_EOL);
    } else {
        printf('Document %s does not exist!' . PHP_EOL, $snapshot->id());
    }
}

上面的代码抛出HTTP错误500.这是我的示例数据库：

错误日志（CentOS 6）显示：

[06-May-2018 22:24:26 UTC] PHP Warning:  Module 'protobuf' already loaded in Unknown on line 0
[06-May-2018 22:24:26 UTC] PHP Warning:  Module 'protobuf' already loaded in Unknown on line 0
[06-May-2018 22:24:27 UTC] PHP Fatal error:  Call to undefined method Google\Protobuf\Timestamp::getFields() in /home/user/public_html/my/vendor/google/gax/src/ApiCore/Serializer.php on line 222
[06-May-2018 22:25:02 UTC] PHP Warning:  Module 'protobuf' already loaded in Unknown on line 0
[06-May-2018 22:25:02 UTC] PHP Warning:  Module 'protobuf' already loaded in Unknown on line 0

我知道Firestore仍处于测试阶段。这是已知问题/限制之一吗？

Answer 1

作为一种解决方法，我能够从嵌套文档中获取数据，如下所示：

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