How to retrieve all data from a subcollection of all document

Question

I am using Flutter for my app. I want to retrieve all document (not only for one user) of a subcollection and display in the app. Each user has a subcollection of Baggage and a subcollection of Car.

This is how I store in Firebase:

String createAt = DateTime.now().millisecondsSinceEpoch.toString();
var ref = FirebaseFirestore.instance.collection('Trips').doc('userId').collection('Baggage').doc(createAt);
FirebaseFirestore.instance.runTransaction((transaction) async {transaction.set(ref, {
    'departure': city1.text, 
    'destination': city2.text, 
    'weight': _weightcontroller.text, 
    'id': userId, 
    'departureDate': date.text, 
    'username': firstname, 
    "timestamp": createAt,
  }); 
});
    
    
var ref1 = FirebaseFirestore.instance
  .collection('Trips')
  .doc('userId')
  .collection('Cars')
  .doc(createAt);

How to retrieve data from subcollection 'Baggage' for all users. If there is best way so structure these data and retrieve easily, please let me know.

Answer 1

Firestore has something called collectionGroup query where you can get all subcollections if they have same collection name. Which can be done as

FirebaseFirestore.instance.collectionGroup('Baggage').get();

This will give you all the documents across all rooms. For reference documentation