Firebase rules: Unknown variable '$uid' - firebase

Question

I want user to only access their own content, except for one child node: common
In common child node I want all signed in users to have access.
I have made the following rules:
{
"rules": {
"$uid": {
".read": "$uid === auth.uid",
".write": "$uid === auth.uid"
},
"common" : {
".read": "auth != null && auth.uid == $uid",
".write": "auth != null && auth.uid == $uid"
}
}
}
Firebase gives me the error:
Error saving rules - Line 8: Unknown variable '$uid'.
The error appears in this line: ".read": "auth != null && auth.uid == $uid",

Answer 1

Based on your question, this is your desired database structure:
{
"userIdA": { // anything here can be written by only userIdA
"name": "Tom", // this is just example data
"location": "London",
/* ... */
},
"userIdB": { // anything here can be written by only userIdB
"name": "Sarah",
"location": "New York",
/* ... */
},
/* ... other user data ... */
"common": { // anything here can be written by signed in users
"data1": "some value",
"data2": "some other value",
}
}
The rules for this structure would be:
{
"rules": {
"common" : {
".read": "auth != null", // logged in users can read
".write": "auth != null" // logged in users can write
},
"$uid": { // $uid will be the value of any key, that isn't listed above it (in this case, anything other than "common")
".read": "$uid === auth.uid", // only the matching user can read
".write": "$uid === auth.uid" // only the matching user can write
}
}
}
Note: This data structure isn't very secure. Allow read/write access to only what you need in your database. With this structure, any user could come along and open up their console and delete everything under "/common". You may consider adding ".validate" rules to make sure certain keys (such as "/common/data1") are only strings.

Answer 2

The $uid must be inside users Document like the example below :
{
"rules": {
"users": {
"$uid": {
".read": "$uid === auth.uid",
".write": "$uid === auth.uid"
}
}
}
}