Access a node/path by two specific users only - firebase

Question

I've read up and tried denormalization and while it makes perfect sense for cases like comments/messages that are accessible publicly while a user can only write to his own path/node, I'm having a hard time restricting ".read" rule to the user/owner AND to another user/admin. My use case doesn't publicly post all messages. To expound, for example, messages by user1 are only readable by user1 and admin, while still retaining write-only to user1.
How is this achieved? In security rules, I tried:
"messages": {
".read": "auth !== null",
".write": "auth !== null",
"$message": {
".read": "data.child('userID').val() === auth.uid"
}
}
OR
"messages": {
".write": "auth !== null",
"$message": {
".read": "data.child('userID').val() === auth.uid"
}
}
OR
"messages": {
"$user_id": {
".read": "auth.uid === $user_id"
".write": "auth.uid === $user_id"
}
}
While the last one does restrict reading and writing to the authenticated user, say user1, I had no luck getting the admin user to get all users' messages. The first, I can't circumvent the cascading/top-down rule.
I'm a firebase newbie so if this is really simple to do, I appreciate any helping hand.
Thanks!

Answer 1

This depends on how you define an administrator.
Let's first say that your admin is a known user with uid SOF.
"messages": {
"$user_id": {
".read": "auth.uid === $user_id || auth.uid == 'SOF'"
".write": "auth.uid === $user_id"
}
}
If you want the administrator to be configurable, you'll probably store their ID somewhere else, say in a node called administrators:
administrators
SOF: true
Frank: true
messages
...
In that case your can check if the current uid is either in the current node or it exists in the list of administrators:
"messages": {
"$user_id": {
".read": "auth.uid === $user_id || root.child('administrators/'+auth.uid).exists()"
".write": "auth.uid === $user_id"
}
}
Update
Since you want administrators to be able to read all messages, you'll end up with:
"messages": {
".read": "root.child('administrators/'+auth.uid).exists()",
"$user_id": {
".read": "auth.uid === $user_id",
".write": "auth.uid === $user_id"
}
}