Migrating from Realtime Database to Firestore in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize Firestore in your Firebase project.

Firebase

const db = firebase.[1]();

Drag options to blanks, or click blank then click option'

Afirestore

Bdatabase

Cstorage

Dauth

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'database' instead of 'firestore' will initialize Realtime Database, not Firestore.

Using 'storage' or 'auth' are unrelated services.

To use Firestore, you call firebase.firestore() to get the Firestore instance.

2fill in blank

medium

Complete the code to add a new document to a Firestore collection named 'users'.

Firebase

db.collection('users').[1]({ name: 'Alice', age: 30 });

Drag options to blanks, or click blank then click option'

Aget

Bset

Cupdate

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' requires a document reference, not a collection.

Using 'update' only modifies existing documents.

Using 'get' retrieves data, not adds it.

The add method adds a new document with an auto-generated ID to the collection.

3fill in blank

hard

Fix the error in the Firestore query to get documents where age is greater than 25.

Firebase

db.collection('users').where('age', '[1]', 25).get();

Drag options to blanks, or click blank then click option'

A>

B<=

C=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' will only find users exactly 25 years old.

Using '<=' or '!=' will not match the requirement.

The operator '>' is used to filter documents where the field is greater than the given value.

4fill in blank

hard

Fill both blanks to update a user's age and merge the data without overwriting the entire document.

Firebase

db.collection('users').doc('user123').[1]({ age: 31 }, { [2]: true });

Drag options to blanks, or click blank then click option'

Aset

Bupdate

Cmerge

Doverwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'update' alone works but does not accept merge options.

Using 'overwrite' is not a Firestore method.

Using 'merge' alone is not a method.

Using set with { merge: true } updates fields without replacing the whole document.

5fill in blank

hard

Fill all three blanks to migrate a Realtime Database listener to Firestore snapshot listener.

Firebase

db.collection('users').[1]((snapshot) => {
  snapshot.docs.forEach(doc => {
    console.log(doc.[2](), doc.[3]);
  });
});

Drag options to blanks, or click blank then click option'

AonSnapshot

Bdata

Cid

Dget

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'get' instead of 'onSnapshot' will not listen for realtime changes.

Accessing document data as a property instead of calling data() causes errors.

Using 'data' as a property instead of a method.

onSnapshot listens for realtime updates. doc.data() gets document data, and doc.id gets the document ID.

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