Add an item to a list in Firebase Database

Question

I have the following Firebase database structure. uIds is a type of List<String>. I am trying to add another uId under uIds with an incremented index. setValue() and updateChildren() would require me to retrieve existing data, and push() will add an item with a randomly generated string as a key instead of an incremented index. Is there a simpler way that does not require to retrieve the existing data? Thanks!

  "requests" : {
    "request001" : {
      "interests" : [ "x" ],
      "live" : true,
      "uIds" : [ "user1" ]  // <---- from this
    },
    "request002" : {
      "interests" : [ "y" ],
      "live" : true,
      "uIds" : [ "user2" ]
    }
  }

--------------------------------

Edit:

Sorry for the unclarity. Let me elaborate to make it clear. Say I have the above database and want to update it to the following.

  "requests" : {
    "-KSVYZwUQPfyosiyRVdr" : {
      "interests" : [ "x" ],
      "live" : true,
      "uIds" : [ "user1", "user2" ]   // <--- to this
    },
    "-KSl1L60g0tW5voyv0VU" : {
      "interests" : [ "y" ],
      "live" : true,
      "uIds" : [ "user2" ]
    }
  }

ishmaelMakitla's suggestion, mDatabase.child("requests").child("request001").setValue(newRequest), will overwrite the "request001" with "newRequest". So I should retrieve the existing data of "request001" and add "user2" to the list uIds. It will be something like this:

mDatabase.child("requests").child("request001").addListenerForSingleValueEvent(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        Request newRequest = dataSnapshot.getValue(Request.class);
        newRequest.uIds.add("user2");
        mDatabase.child("requests").child("request001").setValue(newRequest);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}

});

But I am wondering if this process is necessary since what I am trying to do is simply to add one item to the list uIds.

Answer 1

The Firebase documentation on creating data that scales proposes that you use a different data structure:

"requests" : {
  "-KSVYZwUQPfyosiyRVdr" : {
    "interests" : { "x": true },
    "live" : true,
    "uIds" : { 
      "user1": true, 
      "user2": true 
    }
  },
  "-KSl1L60g0tW5voyv0VU" : {
    "interests" : { "y": true },
    "live" : true,
    "uIds" : { 
      "user2": true 
    }
  }
}

Here are a few of the reasons why this data structure works better:

• each uid can now automatically only appear once. We've essentially modeled our data as a set, instead of using an array.
• adding an item is now as easy as ref.child("uUids").child("user3").setValue(true)
• you can now check if a uid exists in your security rules.

I have started re-iterating to myself: whenever you find yourself doing array.contains("xyz"), you should probably be using a set instead of an array. The above mapping with "key": true is an implementation of a set on Firebase.

Efficiency

Some people may think arrays are a more efficient way of storing the data, but in the case of Firebase that is not true:

What you see:

"uIds" : [ "user1", "user2" ]

What Firebase stores:

"uIds" : {
  "0": "user1",
  "1": "user2"
}

So storing a set is pretty much the same:

"uIds" : { 
  "user1": true, 
  "user2": true 
}

Answer 2

Not sure what you mean when you say setValue, etc require you to retrieve existing data. The basic flow for inserting new record is as follows:

private DatabaseReference mDatabase;
// get reference to your Firebase Database.
mDatabase = FirebaseDatabase.getInstance().getReference();
//and here you add a new child to your 'requests' collection
//I am assuming you have a Request model like this..
Request newRequest = new Request(some-params);
mDatabase.child("requests").child(someRequestId).setValue(newRequest);

You can take a look at basic usage guide for Saving Data on Android Firebase.

Update: Following your comment - I think what you are looking to do can be achieved like this: You use the push() method which generates a unique ID every time a new child is added to the specified Firebase reference:

Firebase newRequestRef = mDatabase.child("request").push();
newRequestRef.setValue(newRequest);

This should do it.

I hope this helps.