Migrating from Realtime Database to Firestore in Firebase - Mini Project: Build & Apply

Migrating from Realtime Database to Firestore

📖 Scenario: You have a Firebase Realtime Database storing user profiles. You want to move this data to Firestore for better querying and scalability.

🎯 Goal: Build a simple Firebase script that reads user data from Realtime Database and writes it into Firestore with the same structure.

📋 What You'll Learn

Create a reference to the Realtime Database users node

Create a Firestore collection reference for users

Read all user data from Realtime Database

Write each user document into Firestore

💡 Why This Matters

🌍 Real World

Many Firebase projects start with Realtime Database and later migrate to Firestore for advanced querying and scalability.

💼 Career

Understanding how to migrate data between Firebase databases is useful for cloud engineers and developers working with Firebase backend services.

1

Setup Firebase Realtime Database reference

Create a variable called rtdbRef that references the path 'users' in the Firebase Realtime Database using firebase.database().ref('users').

Firebase

# Create a variable called rtdbRef referencing 'users' in Realtime Database
# Your code here

Hint

Use firebase.database().ref('users') to get the reference.

2

Setup Firestore collection reference

Create a variable called firestoreUsers that references the 'users' collection in Firestore using firebase.firestore().collection('users').

Firebase

const rtdbRef = firebase.database().ref('users');
# Create a variable called firestoreUsers referencing 'users' collection in Firestore
# Your code here

Hint

Use firebase.firestore().collection('users') to get the Firestore collection reference.

3

Read data from Realtime Database and prepare for migration

Use rtdbRef.once('value') to read all user data from Realtime Database. Inside the callback, get the snapshot value into a variable called usersData.

Firebase

const rtdbRef = firebase.database().ref('users');
const firestoreUsers = firebase.firestore().collection('users');

// Read all users data from Realtime Database
# Your code here

Hint

Use once('value') and snapshot.val() to get the data.

4

Write each user to Firestore

Inside the rtdbRef.once('value').then() callback, use a for...in loop with variable userId to iterate over usersData. For each userId, write the user data to Firestore using firestoreUsers.doc(userId).set(usersData[userId]).

Firebase

const rtdbRef = firebase.database().ref('users');
const firestoreUsers = firebase.firestore().collection('users');

rtdbRef.once('value').then(snapshot => {
  const usersData = snapshot.val();
  // Loop over usersData and write each user to Firestore
  # Your code here
});

Hint

Use a for...in loop and firestoreUsers.doc(userId).set() to write 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