无法使用PHP从具有Cloud Firestore的嵌套集合中获取文档

根据Google的官方文档，我无法检索嵌套集合中的文档，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。以下是我的示例数据库：

错误日志（CentOS 6）显示：

---

我知道Firestore仍处于测试阶段。这是已知的问题/限制之一吗？

作为一种解决方法，我可以从嵌套文档中获取数据，如下所示：

$db = new FirestoreClient([
    'projectId' => $projectId,
]);
//$citiesRef = $db->collection('cities'); //working original example
$citiesRef = $db->collection('countries')->document('USA')->collection('cities');
$documents = $citiesRef->select(['capital','country','name','population','state'])->documents();
foreach ($documents as $document) {
    if ($document->exists()) {
        printf('Document data for document %s:' . PHP_EOL, $document->id());
        //print_r($document->fields()); //this no longer works
        print_r($document->data());
        printf(PHP_EOL);
    } else {
        printf('Document %s does not exist!' . PHP_EOL, $snapshot->id());
    }
}

---

我从Google的示例中逐字复制了这段代码。请注意，行

printf（'文档%s不存在！'.PHP_EOL，$snapshot->id（））else

闭包中的code>引用未定义的变量

$snapshot

。

$db = new FirestoreClient([
    'projectId' => $projectId,
]);
//$citiesRef = $db->collection('cities'); //working original example
$citiesRef = $db->collection('countries')->document('USA')->collection('cities');
$documents = $citiesRef->select(['capital','country','name','population','state'])->documents();
foreach ($documents as $document) {
    if ($document->exists()) {
        printf('Document data for document %s:' . PHP_EOL, $document->id());
        //print_r($document->fields()); //this no longer works
        print_r($document->data());
        printf(PHP_EOL);
    } else {
        printf('Document %s does not exist!' . PHP_EOL, $snapshot->id());
    }
}