How to Use `array_contains` in Firestore Security Rules for Authorization Checks?

How can I use the array_contains operator in Firestore security rules to determine if a user is authorized to access specific documents?

allow read, write: if request.auth != null && request.auth.uid in resource.data.allowedUsers;

In Firestore, security rules are critical for defining who can access specific data. The `array_contains` operator allows you to verify if an array in your document contains a specific value (like a user ID), making it a powerful tool for setting up authorization checks.

// Firestore Security Rule Example
match /projects/{projectId} {
  allow read, write: if request.auth != null && request.auth.uid in resource.data.allowedUsers;
}

Causes

• The need for secure authorization mechanisms in applications accessing Firestore.
• Creating data structures in Firestore that involve user IDs stored as arrays.

Solutions

• Define your document structure to include arrays of user IDs who are allowed to access or modify the document.
• Implement Firestore security rules that utilize the `array_contains` operator to check if a user's UID is present in the allowed users' array.

Mistake: Not ensuring the `allowedUsers` array is defined in every document.

Solution: Always check if `allowedUsers` exists before using `array_contains` to prevent runtime errors.

Mistake: Assuming `array_contains` works with non-array fields.

Solution: Confirm that the field you're checking with `array_contains` is indeed an array.