I love Firebase, but the Swift code in the iOS guide is of really low quality. For example (https://firebase.google.com/docs/database/ios/save-data#dele...):

    if currentData.value != nil, let uid = FIRAuth.auth()?.currentUser?.uid {
        var post = currentData.value as! [String : AnyObject]
        var stars : Dictionary<String, Bool>
        stars = post["stars"] as? Dictionary<String, Bool> ?? [:]
        // ...
    }

What this should really be:

    guard let post = currentData.value as? [String : AnyObject],
    uid = FIRAuth.auth()?.currentUser?.uid else { return FIRTransactionResult.successWithValue(currentData) }

        let stars = post["stars"] as? [String : Bool] ?? [:]
        // ...
    }