如果我具有下一个结构，如何从Firebase实时数据库中检索所有数据？

Question

I have an app where all authenticated users can post articles(title, content, username, date). And I have the structure like this:

Where I have node posts and there I have posts from different users. For example, I have there user ID and that user has different posts which separated by push() method key. So, how to retrieve all data(all posts) from this structure? I have the next code but I get null :

mDatabase.addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
        if (dataSnapshot.child("posts").exists()) {
            for (DataSnapshot dataSnapshot1 : dataSnapshot.child("posts").getChildren()) {
                Post post = dataSnapshot1.getValue(Post.class);
                mAdapter.addPost(post);
            }
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        showInformationDialog("Something went wrong! [initializing the list]");
    }
});

And my model class:

public class Post {

    private String title;
    private String content;
    private String author;
    private String date;

    public Post() {
        // default constructor
    }

    public Post(String title, String content, String author, String date) {
        this.title = title;
        this.content = content;
        this.author = author;
        this.date = date;
    }

    public String getTitle() {
        return title;
    }

    public String getContent() {
        return content;
    }

    public String getAuthor() {
        return author;
    }

    public String getDate() {
        return date;
    }
}

Answer 1

Adding a listener on the root node isn't a good practice at all, meaning that you'll be notified for every change thas is happening in your entire database. It's a waste of bandwith and resources. To solve this, attach the listener two levers deeper, like in the following lines of code:

String uid = FirebaseAuth.getInstance().getCurrentUser().getUid();
DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("posts").child(uid);
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            Post post = ds.getValue(Post.class);
            Log.d(TAG, post.getAuthor());

            //Add the post object to a list and pass it to the adapter
        }
    }

    @Override
    public void onCancelled(@NonNull DatabaseError databaseError) {
        Log.d(TAG, databaseError.getMessage()); //Don't ignore errors!
    }
};
uidRef.addListenerForSingleValueEvent(valueEventListener);

The output in your logcat will be:

danchik_kilmez

Edit:

There was nothing in your question that indicates that you want to get all posts of all users, however to achieve this you need to make a change in your database structure to allow you perform such a query. Using your actual database schema, it's not possible. To solve this, you should duplicate the data. This practice is called denormalization and is a common practice when it comes to Firebase. For a better understanding, I recommend you see this video, Denormalization is normal with the Firebase Database .

Also, when you are duplicating data, there is one thing that need to keep in mind. In the same way you are adding data, you need to maintain it. With other words, if you want to update/detele an item, you need to do it in every place that it exists.

This being said, you should create another node named allPosts , which will hold all posts of all users, like in the following schema example:

Firebase-root
   |
   --- allPosts
         |
         --- postId
              |
              --- //post details
              |
              --- uid: "eNAS ... tfz2"

If you add a listener on posts node, you'll be able to get all post of all users.

Answer 2

You Can simply Use FirebaseRecyclerOptions To Retrieve all The data.

FirebaseRecyclerOptions<Post> options = new FirebaseRecyclerOptions.Builder<Post>()
                .setQuery(YourRef,Post.class)
                .build();

        FirebaseRecyclerAdapter<Post,YAdapterHolder>adapter = new
                FirebaseRecyclerAdapter<Post,YAdapterHolder>(options) {
                    @Override
                    protected void onBindViewHolder(@NonNull final YAdapterHolder holder, final int position, @NonNull Post model) {
                        String userIds = getRef(position).getKey();
                        assert userIds != null;
                        YourRef.child(userIds).addValueEventListener(new ValueEventListener() {
                            @Override
                            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                                if (dataSnapshot.getChildrenCount()!=0){
                         //example value retrive
                                  Long data = dataSnapshot.child("data").getValue(Long.class);
                                String data1 = dataSnapshot.child("data1").getValue(String.class);

                                }


                            }

                            @Override
                            public void onCancelled(@NonNull DatabaseError databaseError) {

                            }
                        });




                    }

                    @NonNull
                    @Override
                    public YAdapterHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {
                        View view = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.pendinglistmodel,viewGroup,false);
                        return new YAdapterHolder(view);
                    }
                };