title: Firestore -- Get only the changed documents in a large synced collection
body: <p>I've read all of the questions below, and cannot find anything in the docs to describe how to sync a collection and receive <em>only</em> changed documents from a collection. I've got over 500 documents in my synced collection (using <a href="https://redux-saga-firebase.js.org/guides/syncing-data" rel="nofollow noreferrer">redux-saga-firebase syncCollection</a>) but usually only about 100 of them ever change. At any given time, even fewer will change, but I'm currently getting <em>all 500+</em> documents back when even one changes, which results in 500+ reads. Not ideal, and will cost me at scale.</p>
  
  <p>Here is my code using redux-saga-firebase, but a vanilla firebase code answer would also suffice. How can get a synchronized response that sends <em>only</em> the changed documents in the collection?</p>
  
  <pre><code>export function* getPlayersFromDb() {
    yield fork(
      rsf.firestore.syncCollection,
      'players',
      { successActionCreator: response =&gt; ({
              type: t.GET_PLAYERS_FROM_DB_SUCCESS,
              payload: [...response],
          }),
          failureActionCreator: () =&gt; ({
              type: t.GET_PLAYERS_FROM_DB_FAILURE,
              payload: [],
          }),
          transform: response =&gt; messageTransformer(response),
      }
  );
  </code></pre>
  
  <p>}</p>
  
  <p><a href="https://stackoverflow.com/questions/50089835/does-firestore-sync-only-diff">Does Firestore sync only diff?</a></p>
  
  <p><a href="https://stackoverflow.com/questions/54096606/realm-syncing-with-large-collection-on-firestore-architectural-questions-iss">Realm syncing with large collection on Firestore - architectural questions / issues</a></p>
  
  <p><a href="https://stackoverflow.com/questions/52700648/how-to-avoid-unnecessary-firestore-reads-with-cache">How to avoid unnecessary Firestore reads with Cache</a></p>
  
  <p><a href="https://stackoverflow.com/questions/56682203/what-exactly-does-firestore-synchronization-do">What exactly does Firestore synchronization do?</a></p>