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

Question

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.

Answer 1

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: "paul@harddaysnight.com", 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'"