Migrating from Realtime Database to Firestore in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Firestore Data Model Differences

When migrating from Realtime Database to Firestore, which of the following best describes a key difference in how data is structured?

A Firestore uses a flat JSON tree, while Realtime Database stores data in tables.

B Both Firestore and Realtime Database use the same JSON tree structure but with different security rules.

C Firestore stores data in documents and collections, while Realtime Database uses a large JSON tree.

D Realtime Database stores data in documents and collections, while Firestore uses a large JSON tree.

💡 Hint

Think about how Firestore organizes data compared to Realtime Database.

Firestore organizes data into documents and collections, which allows for more structured and scalable data storage. Realtime Database uses a single large JSON tree.

❓ Architecture

intermediate

Choosing Firestore Security Rules for Migrated Data

After migrating data from Realtime Database to Firestore, which security rule approach is recommended to maintain similar access control?

A Use Firestore rules that check document fields and user authentication to control access at the document level.

B Reuse Realtime Database rules directly in Firestore without changes.

C Disable all Firestore security rules temporarily to avoid access issues.

D Use Firestore rules only at the collection level without checking document fields.

💡 Hint

Consider how Firestore rules differ from Realtime Database rules in granularity.

Firestore security rules operate at the document level and can check fields and authentication to control access precisely. Realtime Database rules cannot be reused directly.

❓ Configuration

advanced

Firestore Query Behavior After Migration

Which Firestore query will correctly retrieve all documents from the 'users' collection where the 'age' field is greater than 25?

const usersRef = firestore.collection('users');
const query = ???;

A usersRef.get('age' > 25)

B usersRef.filter('age', '>', 25)

C usersRef.query('age' > 25)

D usersRef.where('age', '>', 25)

💡 Hint

Recall the Firestore method to filter documents by field values.

Firestore uses the where method to filter documents by field values. Other methods like filter or query do not exist.

❓ service_behavior

advanced

Handling Offline Support Differences

After migrating to Firestore, which statement correctly describes offline data behavior compared to Realtime Database?

A Firestore disables offline support by default and requires extra configuration to enable it.

B Realtime Database has built-in offline support, but Firestore does not support offline mode.

C Both Firestore and Realtime Database require manual caching for offline support.

D Firestore provides built-in offline support with automatic synchronization when online, unlike Realtime Database which requires manual handling.

💡 Hint

Think about how Firestore manages offline data synchronization.

Firestore disables offline support by default (on web SDK) and requires extra configuration such as enablePersistence() to enable it, while Realtime Database has built-in offline support automatically.

❓ security

expert

Preventing Unauthorized Data Access Post-Migration

After migrating to Firestore, you want to ensure users can only read their own profile documents stored in the 'users' collection where document IDs match user IDs. Which Firestore security rule snippet correctly enforces this?

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId} {
      allow read: ???;
    }
  }
}

A request.auth.uid == 'admin'

B request.auth != null && request.auth.uid == userId

C request.auth.uid != null

D true

💡 Hint

Consider how to match authenticated user ID with document ID.

The rule allows read only if the user is authenticated and their UID matches the document ID, preventing access to other users' data.

Practice

1. What is the main difference between Firebase Realtime Database and Firestore when migrating data?

easy

A. Realtime Database requires manual backups, Firestore automatically backs up data.

B. Realtime Database uses SQL queries, Firestore uses NoSQL queries.

C. Realtime Database stores data as one large JSON tree, Firestore stores data in documents and collections.

D. Realtime Database supports offline mode, Firestore does not.

Solution

1. Step 1: Understand data structure differences

   Realtime Database organizes data as a single JSON tree, making it less flexible for complex queries.

2. Step 2: Recognize Firestore's document model

   Firestore stores data in documents grouped into collections, allowing more structured and scalable data management.

3. Final Answer:

   Realtime Database stores data as one large JSON tree, Firestore stores data in documents and collections. -> Option C

4. Quick Check:

   Data structure difference = Realtime Database stores data as one large JSON tree, Firestore stores data in documents and collections. [OK]

Hint: Remember: JSON tree vs documents/collections [OK]

Common Mistakes:

• Confusing query languages between databases
• Assuming Firestore lacks offline support
• Thinking backups are automatic in Realtime Database

2. Which of the following is the correct Firebase CLI command to export data from Realtime Database before migration?

easy

A. firebase database:get / > ./backup.json

B. firebase firestore:export ./backup

C. firebase database:backup ./backup.json

D. firebase export realtime ./backup.json

Solution

1. Step 1: Identify the correct CLI command for Realtime Database export

   The Firebase CLI uses firebase database:get / > ./backup.json to export Realtime Database data.

2. Step 2: Confirm the syntax

   The command outputs JSON to stdout, which is redirected to ./backup.json.

3. Final Answer:

   firebase database:get / > ./backup.json -> Option A

4. Quick Check:

   Export command syntax = firebase database:get / > ./backup.json [OK]

Hint: Export Realtime DB with 'database:get / > ./backup.json' command [OK]

Common Mistakes:

• Using firestore export command for Realtime Database
• Incorrect command syntax
• Confusing backup and export commands

3. Given this snippet migrating data from Realtime Database JSON to Firestore documents, what will be the Firestore document ID for user 'user123'?

const userData = {
  "user123": {"name": "Alice", "age": 30},
  "user456": {"name": "Bob", "age": 25}
};

for (const userId in userData) {
  firestore.collection('users').doc(userId).set(userData[userId]);
}

medium

A. The user's name, e.g., "Alice"

B. A random auto-generated ID

C. "users"

D. "user123"

Solution

1. Step 1: Analyze the loop and document ID assignment

   The loop uses doc(userId), so the document ID is set explicitly to the key, e.g., "user123".

2. Step 2: Confirm document ID is not auto-generated

   Since doc(userId) is used, Firestore will not auto-generate an ID.

3. Final Answer:

   "user123" -> Option D

4. Quick Check:

   Document ID = userId key = "user123" [OK]

Hint: doc(userId) sets document ID explicitly [OK]

Common Mistakes:

• Assuming Firestore auto-generates IDs always
• Confusing collection name with document ID
• Using user name as ID mistakenly

4. You tried migrating data using a script but Firestore documents are empty after migration. What is the most likely cause?

medium

A. You used the wrong collection name in Firestore.

B. You forgot to await the asynchronous set() calls in the migration script.

C. You exported data from Firestore instead of Realtime Database.

D. You did not enable Firestore in the Firebase console.

Solution

1. Step 1: Understand asynchronous Firestore writes

   Firestore's set() method returns a promise that must be awaited to complete the write.

2. Step 2: Identify missing await causes empty documents

   If set() is not awaited, the script may finish before writes complete, resulting in empty documents.

3. Final Answer:

   You forgot to await the asynchronous set() calls in the migration script. -> Option B

4. Quick Check:

   Await async writes = You forgot to await the asynchronous set() calls in the migration script. [OK]

Hint: Always await Firestore writes in migration scripts [OK]

Common Mistakes:

• Ignoring async nature of Firestore writes
• Assuming collection name mismatch causes empty docs
• Confusing export source databases

5. You want to migrate nested Realtime Database data with users and their posts to Firestore. Which approach best preserves the data structure and supports efficient queries?

hard

A. Store users as documents in 'users' collection and posts as subcollections under each user document.

B. Flatten all data into a single collection with combined user-post IDs as document IDs.

C. Store all data in one large Firestore document per user including all posts as nested arrays.

D. Migrate posts to a separate collection without linking to users.

Solution

1. Step 1: Understand Firestore data modeling best practices

   Firestore supports subcollections to model one-to-many relationships efficiently.

2. Step 2: Choose structure preserving hierarchy and query efficiency

   Storing posts as subcollections under user documents keeps data organized and allows targeted queries.

3. Final Answer:

   Store users as documents in 'users' collection and posts as subcollections under each user document. -> Option A

4. Quick Check:

   Use subcollections for nested data = Store users as documents in 'users' collection and posts as subcollections under each user document. [OK]

Hint: Use subcollections for nested related data in Firestore [OK]

Common Mistakes:

• Putting all data in one large document causing size limits
• Flattening data losing natural hierarchy
• Separating posts without user linkage