how to check favorite item document is exist from firestore

问题

I need when user open the meal page see if that meal in favorite or not by making the favorite icon color to red but i can't check if the meal document is in firestore or not.

I've tried to put this code in init state but it doesn't work i check icon color from isFavorite boolean.

    firestore
    .collection('favorites')
    .where('mealId', isEqualTo: widget.mealId)
    .where('userId', isEqualTo: userID)
    .getDocuments().then((f) {
  if(f.documents.isNotEmpty) {
    setState(() {
      isFavorite = true;
    });
  } else {
    setState(() {
      isFavorite = false;
    });
  }
});

this code make all of meals have a red favorite icon.

来源：https://stackoverflow.com/questions/56736122/how-to-check-favorite-item-document-is-exist-from-firestore