Enforce Field Uniqueness HTTP

Made by Tomas Piaggio

Enforces uniqueness on a specified field of a document written to a specified Cloud Firestore collection, using a HTTP Callable function.

Works with
Cloud Firestore
0.1.6 | Source code
Tomas Piaggio

How this extension works

Use this extension to ensure uniqueness on a specified field (for example, username) of a document within a specified to a Cloud Firestore collection (for example, users).

This extension allows you to write a document to your specified Cloud Firestore collection using a Callable HTTP function. If you add a string to a specified field in the document within that collection, this extension:

  • Gets the type of event sent to the callable function and the string value of the specified field.
  • Writes the document to the specified collection if the value of the specified field is unique.
  • Creates or deletes a document with that value as it’s key on a separate specified aux collection

How to use

This extension works using a callable function to write documents to the collection, instead of writing directly from the client app. The goal of this extension is to maintain documents on an aux collection, each document having the unique field value as the key, and thus checking for uniqueness of these documents using a transaction on the server.

In the following example, we’ll be using users and username as the collection and field respectively, but it could be any field name and collection name.

const data = {
  change: "CREATE",
  collection: "users",
  fieldName: "username"
  document: {

const callable = firebase.functions().httpsCallable("ext-field-uniqueness-http-fieldUniqueness");
.then((result) => {
  // User created successfully
.catch((error) => {
  // Username already taken

You should also indicate in your security rules that the collection, in this example called users, should have allow write: if false; as a security rule, since we don’t want to allow the client to write to this collection, only the HTTP callable function should be writing to it.

Additional setup

Before installing this extension, make sure that you’ve set up a Cloud Firestore database in your Firebase project.


To install an extension, your project must be on the Blaze (pay as you go) plan

  • You will be charged a small amount (typically around $0.01/month) for the Firebase resources required by this extension (even if it is not used).
  • This extension uses other Firebase and Google Cloud Platform services, which have associated charges if you exceed the service’s no-cost tier:
    • Cloud Firestore
    • Cloud Functions (Node.js 10+ runtime. See FAQs)