How to get data from Firebase Firestore?

问题

In this code I added "Comment" in firestore this is not problem.

btnAddComment.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                btnAddComment.setVisibility(View.INVISIBLE);
                DocumentReference comment = firestore.collection("Comment").document(postKey);
                String comment_content = editTextComment.getText().toString();
                String uid = currentUser.getUid();
                String uname = currentUser.getDisplayName();
                String uimg = currentUser.getPhotoUrl().toString();
                Comment comm = new Comment(comment_content, uid, uimg, uname);

                comment.set(comm).addOnSuccessListener(new OnSuccessListener<Void>() {
                    @Override
                    public void onSuccess(Void aVoid) {
                        showMessage("Добавлено");
                        editTextComment.setText("");
                        btnAddComment.setVisibility(View.VISIBLE);
                    }
                }).addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        showMessage("Не добавлено: " + e.getMessage());
                    }
                });
            }
        });

And here's how to stitch it:

private void iniRvComment() {
        RwComment.setLayoutManager(new LinearLayoutManager(this));
        DocumentReference docRef = firestore.collection("Comment").document(postKey);
        docRef.collection("Comment").addSnapshotListener(new EventListener<QuerySnapshot>() {
            @Override
            public void onEvent(@Nullable QuerySnapshot documentSnapshot, @Nullable FirebaseFirestoreException e) {
                if (documentSnapshot != null && !documentSnapshot.getDocuments().isEmpty()) {
                    listComment = new ArrayList<>();
                    List<DocumentSnapshot> documents = documentSnapshot.getDocuments();
                    for (DocumentSnapshot value : documents) {

                        Comment comment = value.toObject(Comment.class);
                        listComment.add(comment);
                    }
                    commentAdapter = new CommentAdapter(getApplicationContext(), listComment);
                    RwComment.setAdapter(commentAdapter);
                }
            }
        });
    }

I want to display the "Comment" field with firestore using "postKey" as the key but I can understand how this code works, watched it all on the official Firebase website. I have a "Post" under each post the user leaves a comment, the problem is that something is not displayed.

回答1:

The above code won't work because of your database reference declaration. If you want to retrieve all the comments just provide collection referece.

firestore.collection("Comment").addSnapshotListener(new EventListener<QuerySnapshot>() {
    @Override
    public void onEvent(@Nullable QuerySnapshot documentSnapshot, @Nullable FirebaseFirestoreException e) {
        if (documentSnapshot != null && !documentSnapshot.getDocuments().isEmpty()) {
            listComment = new ArrayList<>();
            List<DocumentSnapshot> documents = documentSnapshot.getDocuments();
            for (DocumentSnapshot value : documents) {

                Comment comment = value.toObject(Comment.class);
                listComment.add(comment);
            }
            commentAdapter = new CommentAdapter(getApplicationContext(), listComment);
            RwComment.setAdapter(commentAdapter);
        }
    }
});

回答2:

As I see in your screenshot, you have a schema that looks like this:

Firestore-root
    |
    --- Comment (collection)
          |
          --- docId
               |
               --- content: "hfjnncjf"

If you want to get all Comment objects, you only need to use the following CollectionReference:

CollectionReference commentRef = firestore.collection("Comment");
commentRef.addSnapshotListener(/* ... */);

What is wrong in your code is the addition of .collection("Comment") call on your commentRef object. What you are doing, you are assuming that you have a subcollection within your commentRef document, which is wrong. You don't have such a subcollection there.

If you want to get a single comment, then you should use the following line of code:

DocumentReference commentRef = firestore.collection("Comment").document(postKey);

But without iterating, because you only get one document.

Edit:

To get the value of your content property and listen for changes in real-time, please use the following lines of code:

FirebaseFirestore firestore = FirebaseFirestore.getInstance();
DocumentReference commentRef = firestore.collection("Comment").document("ZQMqIrx19fftjO3ZeSzkuEw87MY2");
commentRef.addSnapshotListener(new EventListener<DocumentSnapshot>() {
    @Override
    public void onEvent(@Nullable DocumentSnapshot snapshot, @Nullable FirebaseFirestoreException e) {
        if (e != null) {
            Log.w(TAG, "Listen failed.", e);
            return;
        }

        if (snapshot != null && snapshot.exists()) {
            Log.d(TAG, snapshot.getString("content"));
        }
    }
});

The output in your logcat will be:

hfjnncjf

来源：https://stackoverflow.com/questions/60374474/how-to-get-data-from-firebase-firestore