Deleting documents in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when you delete a Firestore document that does not exist?

You try to delete a document in Firestore using its path, but the document does not exist. What is the result?

Firebase

const docRef = firestore.collection('users').doc('nonexistentDoc');
await docRef.delete();

A The delete operation throws an error indicating the document was not found.

B The delete operation retries automatically until the document exists.

C The delete operation creates an empty document at that path.

D The delete operation succeeds silently without error.

💡 Hint

Think about how Firestore handles deletions for documents that are not present.

Deleting a non-existent document in Firestore does not cause an error. The operation succeeds silently because the document is effectively already deleted.

❓ Configuration

intermediate

Which Firestore security rule allows deleting documents only if the user is authenticated?

Choose the Firestore security rule snippet that permits document deletion only when the user is signed in.

Firebase

service cloud.firestore {
  match /databases/{database}/documents {
    match /items/{itemId} {
      allow delete: if <condition>;
    }
  }
}

A request.auth != null

B request.auth == null

C request.auth.uid == 'admin'

D true

💡 Hint

Check how to verify if a user is signed in within Firestore rules.

The condition request.auth != null ensures the user is authenticated before allowing delete operations.

❓ Architecture

advanced

How to efficiently delete all documents in a large Firestore collection?

You have a Firestore collection with thousands of documents. What is the best approach to delete all documents efficiently?

A Use Firestore client SDK to delete documents with a single query.

B Delete the collection directly using a single delete() call on the collection reference.

C Use a batch delete with Firestore Admin SDK, deleting documents in chunks of 500.

D Delete documents one by one in a loop without batching.

💡 Hint

Consider Firestore limits on batch operations and how to handle large data sets.

Firestore does not support deleting a whole collection at once. The recommended way is to delete documents in batches (max 500 per batch) using the Admin SDK for efficiency and to avoid timeouts.

❓ security

advanced

What security risk arises if Firestore delete rules allow all users to delete any document?

What is the main security risk if Firestore rules allow any authenticated user to delete any document in a collection?

A Users can delete documents they do not own, causing data loss.

B Users can read documents but cannot delete them.

C Users can only delete their own documents, no risk involved.

D Users can create new documents but not delete existing ones.

💡 Hint

Think about the principle of least privilege and data ownership.

If delete rules are too permissive, users can remove data they should not control, leading to unauthorized data loss.

🧠 Conceptual

expert

What is the effect of deleting a Firestore document that has subcollections?

You delete a Firestore document that contains subcollections. What happens to the subcollections and their documents?

A The document and all its subcollections and documents are deleted automatically.

B Only the document is deleted; subcollections remain intact and accessible.

C The delete operation fails if subcollections exist under the document.

D The document is deleted and subcollections are archived automatically.

💡 Hint

Consider Firestore's data model and how deletions propagate.

Deleting a document does not delete its subcollections. Subcollections remain and must be deleted separately if desired.