title: firestore onDelete event triggers other delete events
body: <p>I am trying to use the OnDelete trigger for my Cloud Functions on Firestore. I have two collections "alerts" and "logs". The Log object has an "alertId" key. What I'm trying to do is when an Alert is deleted to delete all the corespondent logs using a cloud function.</p>
  
  <p>Something like that:</p>
  
  <pre><code>    exports.deleteLogs = functions.database.instance('my-app').ref('/alerts/{alertId}')
          .onDelete((snap) =&gt; {
              snap.ref('logs',ref =&gt; ref.where('alertId', '==', alertId)).delete();
      });
  </code></pre>