java Firebase数据库如何从1引用和等于2引用中获取一些值以获取其键和值

Question

我有如下所示的用户信息

我想检查commentWriterUid是否等于用户密钥中的1，以获取其信息

我试过这个：

Query query = mDatabaseReference_comments.orderByChild("commentWriterUid").equalTo(FirebaseUtil.getUsersReference().getKey());

        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                userModel = dataSnapshot.getValue(UserModel.class);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        mFirebaseRecyclerAdapter = new FirebaseRecyclerAdapter<CommentModel, CommentViewHolder>(CommentModel.class,
                R.layout.layout_comment,
                CommentViewHolder.class,
                mDatabaseReference_comments) {

            @Override
            protected void populateViewHolder(CommentViewHolder viewHolder, CommentModel model, int position) {

                Glide.with(PostCommentActivity.this)
                        .load(userModel.getUserImageUrl())
                        .into(viewHolder.mCircleImageView_commentWriterImage);

                viewHolder.mTextView_commentWriterName.setText(userModel.getUserName());

                viewHolder.bindToPost(PostCommentActivity.this, model);

            }

        };

但我有以下错误：

03-04 02:24:34.558 31063-31063/com.app E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.NullPointerException
    at com.app.PostCommentActivity$4.populateViewHolder(PostCommentActivity.java:119)
    at com.app.PostCommentActivity$4.populateViewHolder(PostCommentActivity.java:113)

Answer 1

没有人回答我的问题，所以我自己想出来，回答这个问题，这样可能会对其他人有所帮助

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FirebaseDatabase firebaseDatabase = FirebaseDatabase.getInstance();

        final DatabaseReference root_ref = firebaseDatabase.getReference();

        root_ref.child("comments").child("KeKwGa5btKuDYr3yYq5").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                String commentWriterUid = dataSnapshot.child("commentWriterUid").getValue().toString();

                root_ref.child("users").equalTo(commentWriterUid).addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {

                        // Do all stuff you went

                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

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