title: Firestore Aggregation - Fanout and Update Cloud Function
body: <p>I have a document to distribute inside other documents based on an array of IDs inside the original document.</p>
  <p>Original Document;</p>
  <pre><code>name: ABC
  image: https://.....g.png
  duplicateTo: [someID1, someID2, someID3]
  </code></pre>
  <p>How do i distribute this document to documents that has the duplicateTO ID. And if an ID is deleted from original document's duplicateTO array does nodejs' <strong>onUpdate</strong> knows it? Can I also delete the distributed data from the target documents?</p>
  <p>How do we do it? Thank you,</p>