How to read/write from one IOS App set up with firebase, to another firebase database contained in another firebase project? Swift 3

I have a Firebase database connected to my IOS app with the GoogleService-Info.plist. In AppDelegate I configured the App FIRApp.configure(). I could read/write data.

Now, from within this IOS app, I would like to access another FireBase Database brevCustomer. For some reason let dbRef from viewDidLoad has a flag in Xcode saying this 'immutable value dbRef was never used' and the app crashes on the first line in fun startObserving() dbRef.observe(.value, with: { (snapshot: FIRDataSnapshot) in.

Could anyone show how to do the configuration so that I can read/write to brevCustomer database?

EDIT

Please consider the following scenario:

• I have two IOS apps Customer and Worker and two Firebase Projects named CustomerFireBase and WorkerFirebase and I would like them to work in the following way.

• Customer registers with email and password, logs in, makes a booking, and data is saved in CustomerFireBase.

• Worker registers with email and password, logs is, observe WorkerFirebase for value changes or child added
  • read from CustomerFireBase
    • write to CustomerFireBase
    • write to WorkerFirebase

How can I achieve this? Basically, I need to get read/write access from one IOS app configured in the usual way with Firebase, to another Firebase Database contained in another Firebase project.

Class Claim {

  var dbRef:FIRDatabaseReference! //create a reference to Firebase database `brevCustomer`, not the one from .plist file

   override func viewDidLoad() {
       super.viewDidLoad()

     let app = FIRApp(named: "brevCustomer")
     let dbRef = FIRDatabase.database(app: app!).reference().child("Users")
     startObservingDB() // observe the database for value changes
    }

 func startObservingDB() {
   //it crashes on the line below
    dbRef.observe(.value, with: { (snapshot: FIRDataSnapshot) in

        //iterate over each user node child
        for user_child in snapshot.children {
             print(user_child)} 

          }, withCancel: { (Error: Any) in
         print(Error)
       })
   } // end of startObservingDB()
}//end of Claim class



class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

// Use Firebase library to configure APIs for the initial project with .plist file saved in Xcode
FIRApp.configure()


    /** 1. create a Firebase options object to hold the configuration data for the second Firebase Project */
    let secondaryOptions = FIROptions(googleAppID: "1:82424687545:ios:71df5d45218ad27",
                                      bundleID: "com.vivvdaplar.Brev",
                                      gcmSenderID: "8201647545",
                                      apiKey: "AIzaSyCNtyUf2T3UunH6-ci_WyvOqCl_RzXI",
                                      clientID: "8200687545-42vklp94reavi6li6bolhcraoofc6.apps.googleusercontent.com",
                                      trackingID: nil,
                                      androidClientID: nil,
                                      databaseURL: "https://brev-72e10.firebaseio.com",
                                      storageBucket: "com.vivvdaplar.Brev",
                                      deepLinkURLScheme: nil)

    // Configure the app
    FIRApp.configure(withName: "brevCustomer", options: secondaryOptions!) 
       return true
  }
} //end of AppDelegate

Responding the question and comments.

As you know, when a user registers with Firebase, a user account is created on the Firebase server and the user is provided a user id (uid).

A typical design pattern is to have a /users node in Firebase that stores other information about the user, such as a nickname, address or phone number.

We can leverage that /users node to also indicate what kind of user it is; Worker or Client, which would tie into the rest of the app and Firebase so they get to the correct data.

For example

users
  uid_0
    nickname: "John"
    user_type: "Worker"
  uid_1
    nickname: "Paul"
    user_type: "Client"
  uid_2
    nickname: "George"
    user_type: "Worker"
  uid_3
    nickname: "Ringo"
    user_type: "Worker"

As you can see, John, George and Ringo are all workers and Paul is a client.

When the user logs in, the Firebase signIn function will return the users auth data, which contains the uid.

    Auth.auth().signIn(withEmail: "[email protected]", password: "dog",
     completion: { (auth, error) in

        if error != nil {
            let err = error?.localizedDescription
            print(err!)
        } else {
            print(auth!.uid)
            //with the uid, we now lookup their user type from the
            //users node, which tells the app if they are a client
            //or worker
        }
    })

If the app data is divided like this

app
  client_data
     ...
  worker_data
     ...

A simple rule could be set up that verifies the users user_type is Worker for the worker_data node and Client for the client_data node. Here's a pseudo example that will allow a Client user to only access the data in the client_data node (conceptual)

rules 
  client_data
     $user_id
        ".read": "auth != null && root.child(users)
                                      .child($user_id)
                                      .child("user_type") == 'Client'"