Author: Tomas Piaggio (https://github.com/tpiaggio)

Description: Ensures uniqueness on a specified field of a document written to a specified Cloud Firestore collection.

Details: 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 listens to your specified Cloud Firestore collection. If you add a string to a specified field in any document within that collection, this extension:

• Gets the type of event it listened to and the string value of the specified field.
• Creates or deletes a document with that value as it's key on a separate specified aux collection.
• Allows you to decide if you want to check uniqueness for existing documents or only new ones.

This extension works in combination with Firestore Security Rules. 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 Firestore Security Rules.

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 (the .lower() piece is optional in case uniqueness is not case sensitive).

function isUsernameAvailable() {
  return !exists(/databases/$(database)/documents/usernames/$(request.resource.data.username.lower()));
}

function usernameDidNotChange() {
  return request.resource.data.username == resource.data.username;
}

match /users/{userId} {
  allow read: if ...;
  allow create: if isUsernameAvailable();
  allow update: if usernameDidNotChange() || isUsernameAvailable();
  allow delete: if ...;
}

If you want, you can hash the value of the field due to constraints on document IDs. For that, you can select 'Yes' when you're prompted to select if you want to hash the field or not. The Security Rules change slightly, looking like this:

function isUsernameAvailable() {
  let username = hashing.md5(request.resource.data.username).toHexString().lower();
  return !exists(/databases/$(database)/documents/usernames/$(username));
}

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)

Configuration Parameters:

• Cloud Functions location: Where do you want to deploy the functions created for this extension? You usually want a location close to your database. For help selecting a location, refer to the location selection guide.

• Collection path: What is the path to the collection that contains the field that you want to check for uniqueness?

• Field name: What is the name of the field that you want to check for uniqueness?

• Hash field: The value of the unique field will be used as an id for a new document in Firestore. Would you like to hash the value of the field due to constraints on document IDs?

• Aux collection: What is the name of the auxiliar collection where you want to write documents?

Cloud Functions:

• fieldUniqueness: Listens for writes of a specified field to your specified Cloud Firestore collection, then writes a new document using that field's value as the key to a specified aux collection that will serve for checking uniqueness.

• fieldUniquenessBackfill: Searches your specified Cloud Firestore collection for existing documents, then writes the new documents in the aux collection.

Access Required:

This extension will operate with the following project IAM roles:

• datastore.user (Reason: Allows the extension to write to Cloud Firestore.)