How to Append data to an Array in Firebase?

Question

I have a array called subscribedTo in my users node. Now I want to append some push ID's to that array whenever a user is subscribed.

But the push ID are being replaced instead of getting appended.

How can i append the push ID's to the array?

Schema

"tester@gmail,com": {
    "email": "tester@gmail,com",
    "hasLoggedInWithPassword": true,
    "name": "tester",
    "subscribedTo": [
      "-KFPi5GjCcGrF-oaHnjr"
    ],
    "timestampJoined": {
      "timestamp": 1459583857967
    }
  }

CODE

public void onSubscribe(View v) {

        final Firebase firebaseRef = new Firebase(Constants.FIREBASE_URL);

        final HashMap<String, Object> userMap = new HashMap<String, Object>();

        pushIDList.add(PROG_ID);
        userMap.put("/" + Constants.FIREBASE_LOCATION_USERS + "/" + mEncodedEmail + "/subscribedTo",
                pushIDList);

        firebaseRef.updateChildren(userMap, new Firebase.CompletionListener() {
            @Override
            public void onComplete(FirebaseError firebaseError, Firebase firebase) {
                Toast.makeText(ProgramDetail.this, "You are subscribed", Toast.LENGTH_SHORT).show();
            }
        });

    }

Frank van Puffelen · Accepted Answer · 2016-04-16 03:32:28Z

When you call updateChildren() with a map, Firebase takes each key and replaces the object at that location with the value from the map.

The Firebase documentation on updateChildren() says this about it:

Given a single key path like alanisawesome, updateChildren() only updates data at the first child level, and any data passed in beyond the first child level is a treated as a setValue() operation.

So in your case, you are replacing the entire contents of "/" + Constants.FIREBASE_LOCATION_USERS + "/" + mEncodedEmail + "/subscribedTo".

The solution is to either make the PROG_ID part of the key in the map:

userMap.put("/" + Constants.FIREBASE_LOCATION_USERS + "/" + mEncodedEmail + "/subscribedTo/"+PROG_ID, true);
firebaseRef.updateChildren(userMap, ...

Or to simply call setValue() at the lower location in the JSON tree:

firebaseRef.child("/" + Constants.FIREBASE_LOCATION_USERS + "/" + mEncodedEmail + "/subscribedTo/"+PROG_ID).setValue(true);

You'll note that in both cases I got rid of your array in favor of the recommended structure for such a so-called index:

"subscribedTo": {
  "-KFPi5GjCcGrF-oaHnjr": true
},

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