Android 从 Firebase 获取数据

Question

I'd like to build a question system. Each question has a category, for example, questions about painting, about music.

My Firebase schema is like this:

So I'd like to get questions about music, I tried:

mDatabase.child("questions").child("music").addListenerForSingleValueEvent(new ValueEventListener() {

        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            // Get question value
            Questions questions = dataSnapshot.getValue(Questions.class);
            String question = questions.getQuestion();
            Toast.makeText(MainActivity.this, question, Toast.LENGTH_LONG).show();
        }

But my toast is empty. Any ideas why?

I also have Questions.java:

public class Questions {

        public String question;

        public Questions() {
            // Default constructor required for calls to DataSnapshot.getValue(User.class)
        }

        public Questions(String question) {
            this.question = question;
        }

        public String getQuestion(){
            return question;
        }

}

Answer 1

If you notice in your schema, the painting and music are different keys.

You can either modify your schema like

-- questions
   --- key
       --- paintingQuestion
       --- musicQuestion

Your model will turn out to be

public class Questions {

        public String paintingQuestion;
        public String musicQuestions;

        public Questions() {
        }

        //.../

}

Or better way is to modify your schema like this

-- questions
       --- key
           --- question
           --- type
           --- created


public class Questions {
    
            public static final String TYPE_MUSIC = "music"; 

            public String question;
            public long created;
            // Int or string your choice. music, question
            public String type = TYPE_MUSIC; // Int or String.. default is music, 
    
            public Questions() {
            }
    
            //.../
    
    }

Once you have modified your schema, then fetch data from firebase.

Answer 2

When using the following reference:

mDatabase.child("questions").child("music")

You are telling Firebase Realtime Database to return all elements that exist under the following hierarchy:

Firebase-root -> questions -> music

You will always get no elements because such a path doesn't exist. I say that because between the "questions" node and the "music" property there is a child missing, which that ID. There two approaches that can help you solve this.

In the first one, you keep your actual schema, without making any change and use the following lines of code, to get, for example, all music questions:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference questionsRef = rootRef.child("questions");
Query query = questionsRef.orderByChild("music");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String name = ds.child("music").getValue(String.class);
            if (music != null) {
                Log.d(TAG, name);
            }
        }
    }

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

The result in the logcat will be:

What kind of music do you like?
Which is more important to you, music or TV?

The second approach would be to create a POJO class like this:

class Question {
    public String type, name;
}

Using this approach, you need to change the database schema a little bit, as explained below:

 Firebase-root
    |
    --- questions
           |
           --- 84384238423842
           |     |
           |     --- type: "music"
           |     |
           |     --- name: "What kind of music do you like?"
           |
           --- 8rs8842348234
                 |
                 --- type: "music"
                 |
                 --- name: "Which is more important to you, music or TV?"

In this way, the following code is required:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference questionsRef = rootRef.child("questions");
Query query = questionsRef.orderByChild("type").equalTo("music");
ValueEventListener valueEventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            Question question = ds.getValue(Question.class);
            Log.d(TAG, question.name);
        }
    }

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

The result in the logcat will be the same as above.