Firestore - Adding a warning to enable indexing before querying collection group

[vishwarajanand]

Per googleapis/google-cloud-php#2542, user claimed that they cannot get the collectionGroup query return successfully. Upon investigation, it seems we disable indexes on collection group by default: source link to docs.

Changes summary

1. Adding a comment in the sample to ensure users have more clarity for querying collection groups.
2. Linked the source docs page about automatic indexing

Tests

1. Created a document structure within a subcollection

2. Created a single field index within a field of subcollection so that it can be queried

3. Renamed the subcollection, field and value identifiers in this PHP sample and I get the following output.

function query_collection_group_filter_eq(string $projectId): void
{
    // Create the Cloud Firestore client
    $db = new FirestoreClient([
        'projectId' => $projectId,
    ]);

    $results = $db->collectionGroup('subcollection')->where('f1', '==', 'v1');
    foreach ($results->documents() as $document) {
        printf('%s => %s' . PHP_EOL, $document->id(), $document->data()['name']);
    }
}

// Produces the following output:
➜  firestore git:(master) ✗ php src/query_collection_group_filter_eq.php $GOOGLE_PROJECT_ID
subcollection_doc1 => subcollection_doc1
➜  firestore git:(master) ✗

[saranshdhingra]

LGTM