如何从android的fireStore数据库中读取嵌套地图

Question

In the above image, I have a map that contains an arrays list of 2020, 2021, etc. I want to display the arrays data in RecyclerView. How to do that in android?

Also how to retrieve data if a map contains a list of maps. I'm able to get the documents with field names, but struggling with how to get maps with field names and data in that maps.

Answer 1

DocumentSnapshot's getData() method, returns an object of type Map<String, Object> . Since it's a Map, we can simply get the year and then iterate through the corresponding URLs, like in the following lines of code:

FirebaseFirestore rootRef = FirebaseFirestore.getInstance();
CollectionReference subjectRef = rootRef.collection("subject");
DocumentReference politicalScienceRef = subjectRef.document("political science");
politicalScienceRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
        if (task.isSuccessful()) {
            DocumentSnapshot document = task.getResult();
            if (document.exists()) {
                List<String> urls = new ArrayList<>();
                Map<String, Object> data = document.getData();
                Map<String, Object> year = (Map<String, Object>) data.get("year");
                for (Map.Entry<String, Object> entry : year.entrySet()) {
                    List<String> yearUrls = (List<String>) entry.getValue();
                    for (String url : yearUrls) {
                        urls.add(url);
                    }
                }
                for (String url : urls) {
                    Log.d("TAG", url);
                }
            } else {
                Log.d("TAG", "No such document");
            }
        } else {
            Log.d("TAG", "get failed with ", task.getException());
        }
    }
});

In the end, simply pass the urls list to an adapter, and display all those URLs into a RecyclerView.