Use more than one Firebase database in single app - Swift

With the newest version of Firebase you should do:

let filePath = Bundle.main.path(forResource: "My-GoogleService", ofType: "plist")
guard let fileopts = FirebaseOptions.init(contentsOfFile: filePath!)
      else { assert(false, "Couldn't load config file") }
FirebaseApp.configure(options: fileopts)

The proper way to initialize another database is to initialize another app, using the FIROptions constructor, like so:

FIRDatabase().database() // gets you the default database

let options = FIROptions(googleAppID:  bundleID: , GCMSenderID: , APIKey: , clientID: , trackingID: , androidClientID: , databaseURL: "https://othernamespace.firebaseio.com", storageBucket: , deepLinkURLScheme: ) // fill in all the other fields 
FIRApp.configureWithName("anotherClient", options: options)
let app = FIRApp(named: "anotherClient")
FIRDatabase.database(app: app!) // gets you the named other database

Or you can initialize from another named plist rather than a huge constructor:

let filePath = NSBundle.mainBundle().pathForResource("MyCool-GoogleService-Info", ofType:"plist")
let options = FIROptions(contentsOfFile:filePath)