Firestore document model in GCP - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Firestore Document Structure

Which statement correctly describes a Firestore document?

A A document is a collection of documents and cannot contain key-value pairs.

B A document is a flat list of values without keys and cannot contain nested collections.

C A document is a single string value stored in Firestore without any keys.

D A document is a collection of key-value pairs and can contain nested collections.

💡 Hint

Think about how data is organized inside Firestore documents.

Firestore documents store data as key-value pairs and can include nested collections, allowing hierarchical data organization.

❓ Architecture

intermediate

Firestore Collection and Document Relationship

Given a Firestore database, which option correctly describes the relationship between collections and documents?

A Collections contain documents, and documents can contain subcollections.

B Documents contain collections, and collections can contain subdocuments.

C Collections contain only other collections, not documents.

D Documents contain only key-value pairs and cannot contain collections.

💡 Hint

Consider how Firestore organizes data hierarchically.

In Firestore, collections hold documents, and documents can have nested subcollections, allowing multi-level data structures.

❓ Configuration

advanced

Firestore Document Data Types

Which Firestore document data type is not supported directly?

A Timestamp

B Array

C Function

D GeoPoint

💡 Hint

Think about what kinds of data Firestore stores versus what it executes.

Firestore supports data types like Timestamp, Array, and GeoPoint, but it does not support storing executable functions as document fields.

❓ security

advanced

Firestore Document Access Control

Which Firestore security rule correctly restricts read access to documents in the 'users' collection only to the authenticated user with matching user ID?

GCP

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read: if request.auth.uid == userId;
    }
  }
}

A request.auth.uid == userId

B request.auth.token.email_verified == true

C request.auth.uid != userId

D request.time < timestamp.date(2025, 1, 1)

💡 Hint

Think about how to match the authenticated user's ID with the document ID.

The rule allows read access only if the authenticated user's ID matches the document ID, ensuring users can only read their own data.

❓ service_behavior

expert

Firestore Document Write Behavior with Transactions

Consider this Firestore transaction code snippet:

const docRef = firestore.collection('counters').doc('counter1');
await firestore.runTransaction(async (transaction) => {
  const doc = await transaction.get(docRef);
  const newCount = (doc.data()?.count ?? 0) + 1;
  transaction.update(docRef, { count: newCount });
});

What is the expected behavior if two clients run this transaction simultaneously?

A Both transactions will overwrite each other, resulting in only one increment.

B One transaction will fail due to a conflict and retry automatically, ensuring correct count increment.

C Both transactions will increment the count correctly without overwriting each other.

D The transaction will cause a syntax error and not run.

💡 Hint

Think about how Firestore transactions handle concurrent writes.

Firestore transactions detect conflicts and retry automatically, so one transaction will retry if a conflict occurs, ensuring the count increments correctly.