title: Query based on keys of firestore Map
body: <p>I'm writing a cloud function which updates a sibling collection onUpdate. I have the follwong:</p>
  <pre><code>await db.collection(
            `users/${userId}/rosters`).where(
            &quot;athletes.keys&quot;, &quot;array-contains&quot;, bid).get();
  </code></pre>
  <p>where &quot;athletes&quot; is a map attribute in each document in rosters. Does this logic not work? Is there some other way to do it?
  Thanks!!</p>