Firestore：使用指定的UID创建文档，最终使用set（）创建文档，但不更新字段

Question

Consider these lines:

        Map<String, Object> the_data = new HashMap<>();
        the_data.put("login", "Anonymous");
        the_data.put("avatar_is_defined", false);
        the_data.put("amount", 0.0);
        the_data.put("deleted", false);
        the_data.put("can_read_user", true);
        Toast.makeText(this, "Initializing your data... Please wait until the screen appears.", Toast.LENGTH_SHORT).show();
        user_database_model.getReferenceToUser(signed_in_user_uid).set(the_data, SetOptions.mergeFields("")).addOnCompleteList[...]

First time the user signs-in, this set call must be executed and all these fields must be actually set in the document whose UID is signed_in_user_uid .

But, if the user signs-in again ( ie: the document already exists), this set either must be ignored (not executed) (in reality it's impossible, see last paragraph), or its fields must not be updated (that's why I've unsuccessly intended by writing SetOptions.mergeFields("") ).

Is it possible to call a set function saying "Firestore, please create this document and fill it with the provided fields if it doesn't exist ; but if it already exist, just... don't do anything"?

Important: due to Firestore Security Rules , I can't get() the document to know if it exists or not. (in the callback I would have call set only if it doesn't exist). So avoiding to call set (not executing it) isn't possible in reality.

Answer 1

If you only want the user to be able to create their document once, and never update it, you can do so in security rules with:

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{uid} {
      // Applies to writes to nonexistent documents
      allow create: if request.auth.uid != null;

      // Reject writes to existing documents
      allow update: if false;    
    }
  }
}

If you want the client-side code to detect whether the document already exists, and create it if it doesn't, use a transaction .