Firebase Query - Accessing nested object in Firebase Android

Question

I have a firebase object that looks like this

posts: {
  -Kc6CT_CF--kVYApIhD9: {
    -comments: {
       -Kc6CkgQ8-5OztuqHqdS {
         text: "Text here",
         timestamp: 1486179732382,
         user: {
              "username": "user1",
              "email": "[email protected]"
         }
       }
    },
    "lat": "37.8136",
    "lng": "144.9631"
  }
}

And I have a POJO that looks like this:

public class Post {

    private Comment comments;
    private double lat;
    private double lng;
    private String text;
    private String timestamp;
    private Motorist user;


    public Post() {  }

    public Comment getComments() {
        return comments;
    }

    public void setComments(Comment comments) {
        this.comments = comments;
    }

    public double getLat() {
        return lat;
    }

    public void setLat(double lat) {
        this.lat = lat;
    }

    public double getLng() {
        return lng;
    }

    public void setLng(double lng) {
        this.lng = lng;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getTimestamp() {
        return timestamp;
    }

    public void setTimestamp(String timestamp) {
        this.timestamp = timestamp;
    }

    public Motorist getUser() {
        return user;
    }

    public void setUser(Motorist user) {
        this.user = user;
    }

    public static class Comment {

        String text;
        String timestamp;
        Motorist user;

        public String getText() {
            return text;
        }

        public void setText(String text) {
            this.text = text;
        }

        public String getTimestamp() {
            return timestamp;
        }

        public void setTimestamp(String timestamp) {
            this.timestamp = timestamp;
        }

        public Motorist getUser() {
            return user;
        }

        public void setUser(Motorist user) {
            this.user = user;
        }
    }
}

Here's the code where I loop in my posts object:

Map<String, Post> td = new HashMap<String, Post>();
    for(DataSnapshot postSnapshot : dataSnapshot.getChildren()){

        Post post = postSnapshot.getValue(Post.class);

        //for (DataSnapshot commentSnapshot : postSnapshot.child("comments").getChildren()) {
        //    System.out.println(commentSnapshot.getValue());
        //}

        td.put(postSnapshot.getKey(), post);
        Toast.makeText(MapsActivity.this, postSnapshot.getValue().toString(), Toast.LENGTH_SHORT).show();
    }

I am using the addValueEventListener. Now, my problem is I cant get the data in comments object. When I run the debugger my comments object is null. What am I missing? Or is there something wrong with my POJO?

Frank van Puffelen · Accepted Answer · 2017-02-04 15:16:14Z

3

There are a few problems in your code and data structure:

-comments: {
   -Kc6CkgQ8-5OztuqHqdS {
     text: "Text here",
     timestamp: 1486179732382,
     user: {
          "username": "user1",
          "email": "[email protected]"
     }
   }

If that - is indeed in front of comments, please remove it. The node should just be called comments and adding a - prefix just makes it more difficult to handle correctly.

Next up is a problem in your code: under (now) comments you have a list of comments, keyed by a push ID (the keys starting with -K. In your code you've only modeled a single comment:

public class Post {
    private Comment comments;

There is no way that the Firebase Database client can map all child nodes of comments into that single Comment, so it skips it when it reads the JSON. The correct mapping of your JSON structure is:

public class Post {
    public Map<String,Comment> comments;

If you then only want to handle the values of the comments, you can get them with post.comments.values().

answered Feb 4, 2017 at 15:16

Frank van Puffelen

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2 Comments

rbo13 Over a year ago

Thanks @Frank van Puffelen. The - in comment is not necessary, I just copied that from the firebase. I will try to follow your suggestion regarding on my comment. Thank you so much!

rbo13 Over a year ago

Hi @Frank van Puffelen. I have a question. Why is it when I query the comments data from firebase it only returns the last record. When I try the logger, it displays all the comments record. Here's my POJO : Here's my looping to get the posts and as well as the comments... whats wrong with my structure? I am still learning the technology as of now.

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