Firestore security rules: check for not exists(/collection/document)

Question

In firebase firestore security rules, I want to allow a request only if a particular document does not exist. My code is:

match /users/{user_id} {
 allow create: if !exists(/databases/$(database)/merchants/$(request.auth.uid));
}

I am pretty sure the document does not exist but it does not work. Both exists() and !exists() give false somehow, or maybe !exists() raises some error.
I have even tried:

match /users/{user_id} {
 allow create: if exists(/databases/$(database)/merchants/$(request.auth.uid)) == false;
}

Is there any way to make this work?

Answer 1

Are your rules inside the database block? Cause I had a similar problem and was actually putting the rule bellow the database block, and my rule was using the $(database)parameter just like yours

service cloud.firestore {
    match /databases/{database}/documents {
        match /users/{user_id} {
            allow create: if !exists(/databases/$(database)/merchants/$(request.auth.uid));
        } 
    }
}