Firebase，从所有文档中获取arraylist字段

Question

I need to query a collection..this collection "Group" contains many documents, and each of them contains, obviously, a title(the id), an array of partecipants, with field named "partecipant", and a field "numPartecipants",with the total partecipants of each group. I attach a photo of the db:

Now, I can get the "partecipant" field and save it into an array with this code:

public void getPartecipantsList(){
    String email = getEmail();
    final String groupTitle = getTitleBar();
    DocumentReference docRef = db.collection("users").document(email).collection("Group").document(groupTitle);

    docRef.get()
            .addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
                @Override
                public void onComplete(@NonNull Task<DocumentSnapshot> task) {

                        DocumentSnapshot document = task.getResult();

                        //Extracting participants ArrayList from the document
                        for(Object item : task.getResult().getData().values()) {

                            String[] values = String.valueOf(item).replace("[", "").replace("]", "").replace(" ", "").split(",");

                            for (String value : values){
                                    partecipantsArrayList.add(value);
                            }

                        }
                        partecipantsArrayList.remove(String.valueOf("["));
                        partecipantsArrayList.remove(partecipantsArrayList.size() - 1);

But that's work if I know the name of the group. In this case, I want to extract from the "Group" collection each documents, and from each documents the Id and the partecipants array..I think I can't use Task, but I have to use QueryDocumentSnapshots. That's my code.

    public void getPartecipantsList() {
    final ArrayList<String> title = new ArrayList<>();
    String email = getEmail();
    DocumentReference docRef = db.collection("users").document(email);

    docRef.collection("Group")
            .get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {
                        for (QueryDocumentSnapshot document : task.getResult()) {
                            titleString = document.getId();
                            title.add(titleString);

                            String[] values = String.valueOf(document).replace("[", "").replace("]", "").replace(" ", "").split(",");

                                for (String value : values) {
                                    partecipantsArray.add(value);
                                }
                            }
                            partecipantsArray.remove(String.valueOf("["));
                            partecipantsArray.remove(partecipantsArray.size() - 1);

                            SharedPreferences sharedPreferences = getSharedPreferences(SHARED_PREFS, MODE_PRIVATE);
                            SharedPreferences.Editor editor = sharedPreferences.edit();
                            for (int i = 0; i < title.size(); i++) {
                                editor.putInt(title.get(i) + "_size", partecipantsArray.size());
                            }
                            for (int i=0;i<title.size();i++) {
                                Log.v("Array", partecipantsArray.toString());
                                Log.v(title.get(i)+"array", String.valueOf(partecipantsArray.size()));
                            }
                            for (int i = 0; i < partecipantsArray.size(); i++) {
                                editor.putString(titleString + "_name" + i, partecipantsArray.get(i)); }
                        }
                    }


            });


}

And the output is completely wrong:

The output instead should be: title = [New York, Prova, Test] partecipantsArray = [Andrea, Tom, Spencer ........ , Nico, Raul, Lorenzo, Eli]

Answer 1

To get the array from within a single document, only these lines of code are required:

db.collection("users").document(email).collection("Group").document(groupTitle).get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
    @Override
    public void onComplete(@NonNull Task<DocumentSnapshot> task) {
        if (task.isSuccessful()) {
            DocumentSnapshot document = task.getResult();
            if (document.exists()) {
                ArrayList<String> arrayList = (ArrayList<String>) document.get("partecipant");
                //Do what you need to do with your ArrayList
                for (String s : arrayList) {
                    Log.d(TAG, s);
                }
            }
        }
    }
});

The output in your logcat will be:

Nico
Raul
Lorenzo
Eli

If you want to get all arrays from within all documents, please use the following lines of code:

db.collection("users").document(email).collection("Group").get().addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
    @Override
    public void onComplete(@NonNull Task<QuerySnapshot> task) {
        if (task.isSuccessful()) {
            for (QueryDocumentSnapshot document : task.getResult()) {
                if (document.exists()) {
                    ArrayList<String> arrayList = (ArrayList<String>) document.get("partecipant");
                    //Do what you need to do with your ArrayList
                    for (String s : arrayList) {
                        Log.d(TAG, s);
                    }
                }
            }
        }
    }
});

The output will be all partecipants within all documents.