Firebase Realtime Database rules for specific node of structure - swift

Question

Before creating a new user I want to check if creating username property already exists in Firebase Database.
Checking function is:
let databaseRef = Database.database().reference()
databaseRef.child("users").queryOrdered(byChild: "username").queryEqual(toValue: loginRegisterTextField.text).observeSingleEvent(of: .value, with: { (snapshot: DataSnapshot) in
if snapshot.exists() {
print("Login exists")
} else {
print("Login does not exist")
}
})
JSON is:
Rules are for node users:
{
"rules": {
"users" : {
".read": "auth != null",
"$uid" : {
".write": "auth != null && auth.uid == $uid",
}
},
Is it possible to write a rules to check existing of username without a new uid?

Answer 1

There is no way to check for a specific value across a JSON branch in security rules. This has been covered quite a few times before, so I recommend checking some of these search results.
But you can make your query on /users more secure, by only allowing that specific query, and not allowing people to read all of /users. To secure the query you could some something like:
{
"rules": {
"users" : {
".read": "auth != null &&
query.orderByChild == 'username' &&
query.equalTo !== null",
...
This is the first time I've used query.equalTo !== null, so there may be some small mistakes in that part, but the flow should be clear.