Deleting fields in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when you delete a field in a Firestore document?

You have a Firestore document with fields name, age, and city. You run an update to delete the age field. What is the state of the document after this operation?

Firebase

const docRef = firestore.collection('users').doc('user1');
docRef.update({ age: firebase.firestore.FieldValue.delete() });

A The <code>age</code> field is set to null but still exists in the document.

B The <code>age</code> field is removed from the document; other fields remain unchanged.

C The update fails with an error because fields cannot be deleted.

D The entire document is deleted from Firestore.

💡 Hint

Think about what FieldValue.delete() does in Firestore updates.

Using FieldValue.delete() in an update removes that field from the document without affecting other fields.

❓ Configuration

intermediate

Which Firestore update correctly deletes a nested field?

You want to delete the nested field address.zipcode from a Firestore document. Which update command will do this correctly?

A docRef.update({ 'address.zipcode': firebase.firestore.FieldValue.delete() });

B docRef.update({ address: { zipcode: firebase.firestore.FieldValue.delete() } });

C docRef.update({ address: firebase.firestore.FieldValue.delete() });

D docRef.update({ 'address.zipcode': null });

💡 Hint

Consider how Firestore handles dot notation for nested fields in updates.

Using dot notation with FieldValue.delete() deletes only the nested field without affecting other nested fields.

❓ Architecture

advanced

Impact of deleting fields on Firestore indexes

You delete a frequently queried field from many documents in a Firestore collection. What is the impact on Firestore indexes and query performance?

A Firestore requires manual index deletion for fields removed from documents.

B The indexes keep the deleted field entries, causing queries to fail until manually rebuilt.

C Deleting fields does not affect indexes; queries on that field return empty results.

D The indexes automatically update to remove the deleted field entries, improving query performance.

💡 Hint

Think about how Firestore maintains indexes when documents change.

Firestore automatically updates indexes when fields are deleted, removing those entries to keep queries accurate and efficient.

❓ security

advanced

Security rules effect on deleting fields in Firestore

You have a Firestore security rule that allows users to update their documents but denies deleting any fields. What happens if a user tries to delete a field using FieldValue.delete()?

A The field is deleted despite the security rule because delete is a special operation.

B The entire document update fails and the document is deleted.

C The update is rejected by Firestore security rules; the field remains unchanged.

D The field is set to null instead of being deleted.

💡 Hint

Consider how Firestore security rules control update operations.

Firestore security rules enforce permissions on all update operations, including field deletions. If deletion is denied, the update is rejected.

✅ Best Practice

expert

Best practice for deleting multiple fields atomically in Firestore

You need to delete multiple fields from a Firestore document in a single operation to keep data consistent. Which approach is best?

A Use a single <code>update()</code> call with all fields set to <code>FieldValue.delete()</code>.

B Call <code>update()</code> multiple times, each deleting one field sequentially.

C Delete the entire document and recreate it without the unwanted fields.

D Read the document, delete fields locally, then overwrite the entire document with <code>set()</code>.

💡 Hint

Think about atomicity and minimizing network calls.

Using a single update() with multiple FieldValue.delete() calls deletes all fields atomically and efficiently.