title: Firestore Update Sub-Collection Document
body: <p>I would like to update a sub-collection document that I got by sending a <strong>group-query</strong> with Flutter. To my current understanding with a <strong>group-query</strong> I do not really know the parent of a sub-collection.</p>
  
  <p>In order to do so, I need the document id of the parent document. The update query would then look like the following:</p>
  
  <pre><code>collection(collectionName)
  .document(parentDocumentId)
   .collection(subCollectionName)
    .document(subCollectionDocumentId)
     .updateData(someMapWithData);
  </code></pre>
  
  <p>Is it necessary to save the parentDocumentId within the sub-collection document to be able to do such update or is there another way to do so?</p>