Using a number sequence to define Firestore document field value

Question

I want to store the user information in the document with its id(not the document id). I want to generate the id particular formate like XYZ0001, where the last 4 digits will automatically increase whenever a new user adds on. Eg, XYZ0001, XYZ0002 like that. I have tried this...

return firestore
          .collection("xyzData")
          .doc(resp.user.uid)
          .set({
            firstName: newUser.firstName,
            lastName: newUser.lastName,
            email: newUser.email,
            ca_id: "XYZ" + pad(firestore.collection("xyzData").count())
          })
          .then

where pad() is a function which gives me last 4 digits

function pad(n) {
  var s = "000" + n;
  return s.substr(s.length - 4);
}

but this is giving me an error that count() is not a function

Also, I was thinking of one another way where I can read the value of the last id and then increase it by one for the next user, bust won't be able to implement it.

Answer 1

Update on fall 2022

At the Firebase Summit 2022, Firebase announced that now Cloud Firestore supports the count() aggregation query. count() allows you to determine the number of documents in a collection or query.

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Old answer, prior to fall 2022

Note that there is no count() method for a CollectionReference. If you want to know the number of documents in a collection you need to call the asynchronous get() method to get a QuerySnapshot and then use the size property. However, you can not pass a call to get() as a property of an object.

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The solution is to maintain you own counter.

There are two standard ways for that:

Use firebase.firestore.FieldValue.increment()

As explained in the doc you can call firebase.firestore.FieldValue.increment() to increment a specific field of a Firestore document. For example, you could have one document in a counter Collection that you increment and read each time you want to get a new value from the sequence. Depending on your exact use case, you may need to use a Transaction.

Also note that "you can update a single document only once per second".

Distributed counter

If you need to update your counter document more than once per second, you should use a Distributed counter as explained in the doc.