Accessing nested collections and document in Firebase (Flutter) - firebase

Question

If you cannot understand the title I am very sorry. I am new to NoSQL don't know how to address this problem.
i am having a nested collection inside my firebase database.
first collection
clicking on '145'
clicking on 'location'
in the chargepoints method I have hardcoded (.doc('WPHW9669')) instead of that I have to loop through the 145 collection and get the document first. after that I have to go inside that looped document to access the 'location' collection.
this is the code which needs to be modified:
chargePoints() {
_db
.collection('bus')
.doc('Routes')
.collection('145')
.doc('WPHW9669') // this the place where the first iteration have to occur
.collection('location')
.orderBy('updatedTime', descending: true)
.limit(1)
.get()
.then((docs) {
if (docs.docs.isNotEmpty) {
for (int i = 0; i < docs.docs.length; i++) {
LatLng position = LatLng(
docs.docs[i].data()['position']['geopoint'].latitude,
docs.docs[i].data()['position']['geopoint'].longitude);
print(docs.docs[i].data()['position']['geopoint'].latitude);
initMarker(position, docs.docs[i].id);
// notifyListeners();
// initMarker(docs.docs[i].data(), docs.docs[i].id);
}
}
});
}

Answer 1

So I found the answer that I am looking for.
instead of creating a complicated collection, I change my collection to be simple.
new collection URL - 145/(Bus NO)/location/
final result = FirebaseFirestore.instance.collection('145');
busPoint() {
result.get().then((snapshot) {
snapshot.docs.forEach((element) {
chargePoints(element.id.toString());
print(element.id);
});
});
}
the above method will loop through the '145' collection and call the chargePoints() method for each bus no's found(i registered the bus no as the document id).
chargePoints(String element) {
_db
.collection('145')
.doc(element)
.collection('location')
.orderBy('updatedTime', descending: true)
.limit(1)
.get()
.then((docs) {
if (docs.docs.isNotEmpty) {
for (int i = 0; i < docs.docs.length; i++) {
LatLng position = LatLng(
docs.docs[i].data()['position']['geopoint'].latitude,
docs.docs[i].data()['position']['geopoint'].longitude);
initMarker(position, docs.docs[i].id);
}
}
});
}