How to connect to more than one firebase database from an android App

Question

I'm trying to have one project in Firebase that will be responsible for one common thing that all Apps.

That is, I want to create Apps, then have these Apps access a particular Firebase Database of a project.

Looking at the Firebase Android docs, I can't find a way to send data to another firebase database in another project using the following, but where reference is of another project.

DatabaseReference ref = FirebaseDatabase.getInstance().getReference().child("example");
        ref.push().setValue(d).addOnCompleteListener(new OnCompleteListener<Void>() {
            @Override
            public void onComplete(@NonNull Task<Void> task) {
                finish();
            }
        });

Answer 1

You'll need to initialize a second FirebaseApp object with explicit options in your code:

FirebaseOptions options = new FirebaseOptions.Builder()
        .setApiKey("AI...j0")
        .setApplicationId("1:5...e0")
        .setDatabaseUrl("https://myapp.firebaseio.com")
        .build();
FirebaseApp secondApp = FirebaseApp.initializeApp(getApplicationContext(), options, "second app");
FirebaseDatabase secondDatabase = FirebaseDatabase.getInstance(secondApp);
secondDatabase.getReference().setValue(ServerValue.TIMESTAMP);

I got the configuration values from the second project's google-services.json. The API Key is under a property called api_key, the Application ID came from a property called mobilesdk_app_id and the database URL came from a property called firebase_url.

Also see the documentation on using multiple projects in your application.