Firestore create document if it doesn't exist security rule

Question

I was trying to write a rule that if the id of the document doesn\'t exist, then create a new document. My object is:

Message message = new Message(userId, title         


        

Answer 1

You can use the exists() and get() built-in functions to verify if the document exists or to access the document so you can read its data.

In this case, an example would be:

match /messages/{message} {
    allow read; 
    allow create: if !exists(/databases/$(database)/documents/messages/$(request.resource.id));
}

And as @creativecreatorormaybenot suggested, you should not specify allow expressions for update and delete so the data cannot be overwritten.

Here is a link to the documentation:

https://firebase.google.com/docs/firestore/security/rules-conditions#access_other_documents

https://firebase.google.com/docs/reference/rules/rules.firestore