docs: add snippet for a subcollection query

[jlara310]

Add a beginner-level snippet that shows how to query a subcollection.
Similar PR for Android: firebase/snippets-android#447
Note that I added two snippet tags: one for the convention used by the file and one for
the convention used on cloud.google.com

Risk Level

• [ x ] No risk
• Somewhat Risky
• High risk

Pre-submit checklist

• [ x] This PR follows the Google Developer Documentation Style Guidelines
• [ x] This PR uses semantic line breaks

[nohe427]

LGTM 👍

[nohe427]

@thatfiredev -> Do you know why CI/CD might be breaking?

[thatfiredev]

@nohe427 It looks like count() was introduced in FlutterFire Firestore v4.0.0, but CI is using 3.1.11:

lib/snippets/firestore.dart:827:28: Error: The method 'count' isn't defined for the class 'CollectionReference<Map<String, dynamic>>'.
 - 'CollectionReference' is from 'package:cloud_firestore/cloud_firestore.dart' ('../../../../../.pub-cache/hosted/pub.dev/cloud_firestore-3.1.11/lib/cloud_firestore.dart').
 - 'Map' is from 'dart:core'.
Try correcting the name to the name of an existing method, or defining a method named 'count'.

[thatfiredev]

Oh, looks like 3.1.x is coming from pubspec.yaml:

snippets-flutter/packages/firebase_snippets_app/pubspec.yaml

Line 27 in 36812ac

cloud_firestore: ^3.1.8

[nohe427]

@jlara310 - Could you run flutter pub upgrade --major-versions to see if that helps get CI to run successfully?

[jlara310]

@jlara310 - Could you run flutter pub upgrade --major-versions to see if that helps get CI to run successfully?

Done, but I encountered additional errors when trying to run the app locally. Filed bugs here:

• Bug in Firestore count_aggregate_query snippets #24
• Bug in phone_auth_web_with_recaptcha snippet #25
• Cannot build firebase_snippets_app for Android #26

[thatfiredev]

@jlara310 Thanks for filing those. Can you commit your current pubspec files after you ran that command so that CI can pick it up in this PR?

[jlara310]

@jlara310 Thanks for filing those. Can you commit your current pubspec files after you ran that command so that CI can pick it up in this PR?

Yep. Done.

[nohe427]

I am going to merge this since it seems the last few merges were also failing. I will then go through and try and fix the logged bugs. Thanks for making these changes and documenting the new errors. This will give us a great launching off point. @thatfiredev - I will ask you for reviews :)