How to avoid entering a repeated data in firebase documents?

Question

My database is quite simple. A collection and inside, there are documents with user information, including their username.

(I am doing this in flutter and firestore).

What I want is to prevent two users from having the same username.

I am doing this to insert the data.

            FirebaseFirestore.instance.collection('users').add({
                              'username': 'username',
                              'desc': 'some bio',
                              'fieldone': 'aaa',
                              'fieldtwo': 'bbb',
                              // other user info
            });

And to consult the data of a user (by their username).

FirebaseFirestore.instance.collection('users').where('username', isEqualTo: 'username').get();

At the moment I insert the data, what can I do to verify that said username is already in the database? So if the username already exists, return an error and don't let insert it. And if it doesn't exist yet, it can be inserted.

What query can help me for this? I am new to firebase. :(. Thanks.

score 0 · Accepted Answer · answered Mar 04 '22 at 00:37

What query can help me for this?

This can't be accomplished by a single query. You will have to take the steps you outlined already in your question. You will have to first query for documents with that username, then only insert a new one if there were none found.

However, you should be aware that there is a race condition here. If two people are trying at the same time to add the same username, it's possible that they both end up adding a document.

The only way to ensure uniqueness in Firestore is using the document ID. You might want to consider using the ID of the document to store the username (or some version of it, like a hash) so that a race condition can never occur, and that you only have one document with that username.

How to avoid entering a repeated data in firebase documents?

1 Answers1