Writing data (set, update, push) in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when using set() with merge option?

Consider this Firebase Realtime Database operation:

ref.set({name: 'Alice'}, {merge: true})

What is the effect of using set with the merge: true option?

A It throws an error because set() does not support a merge option in Realtime Database.

B It updates only the specified fields in the existing data, preserving other fields.

C It appends the new object as a child node under the reference.

D It replaces the entire data at the reference with the new object, removing any existing fields.

💡 Hint

Think about the difference between set() and update() in Firebase Realtime Database.

In Firebase Realtime Database, the set() method replaces all data at the specified reference. The merge option is not supported here; it is a Firestore feature. To update specific fields without overwriting, update() should be used.

❓ Configuration

intermediate

Which code correctly pushes a new item to a list in Firebase Realtime Database?

You want to add a new message to a list of messages stored under /chat/messages. Which code snippet correctly pushes a new message with text 'Hello'?

A firebase.database().ref('/chat/messages').push({text: 'Hello'})

B firebase.database().ref('/chat/messages').set({text: 'Hello'})

C firebase.database().ref('/chat/messages').update({text: 'Hello'})

D firebase.database().ref('/chat/messages').child('text').push('Hello')

💡 Hint

Remember that push() creates a unique key for each new item in a list.

The push() method adds a new child with a unique key under the specified reference. Using set() or update() directly on the list path would overwrite or update the entire list, not add a new item.

❓ Architecture

advanced

How to atomically update multiple fields in Firebase Realtime Database?

You want to update the user's name and age at /users/user123 in a single operation without overwriting other data. Which approach achieves this atomically?

A Use push() with an object containing name and age fields.

B Use set() with an object containing only name and age fields.

C Use update() with an object containing name and age fields.

D Use set() with merge option set to true.

💡 Hint

Think about which method updates specific fields without replacing the entire node.

The update() method updates only the specified fields at the reference without overwriting other data. It is atomic and efficient for partial updates. set() replaces the entire node, push() adds a new child, and set() with merge is not supported in Realtime Database.

❓ security

advanced

What happens if a user tries to push data without write permission in Firebase Realtime Database?

Assuming Firebase Realtime Database rules deny write access to a user at /posts, what happens when the user runs ref.push({title: 'New Post'})?

A The data is pushed successfully and visible to all users.

B The data is pushed but only visible to the user who pushed it.

C The push() call silently fails without error.

D The push() call throws a permission denied error.

💡 Hint

Consider how Firebase enforces security rules on write operations.

Firebase Realtime Database enforces security rules on every write. If the user lacks write permission, the push() operation fails and returns a permission denied error.

✅ Best Practice

expert

Which approach minimizes data loss risk when updating nested data in Firebase Realtime Database?

You need to update a deeply nested field /users/user123/profile/age without affecting other profile data. Which method is best to minimize risk of data loss?

A Use set() on <code>/users/user123/profile</code> with the entire profile object including the updated age.

B Use update() on <code>/users/user123/profile</code> with only the age field.

C Use set() on <code>/users/user123/profile/age</code> with the new age value.

D Use push() on <code>/users/user123/profile/age</code> with the new age value.

💡 Hint

Think about partial updates and avoiding overwriting unrelated data.

Using update() on the profile node with only the age field updates that field without overwriting other profile data. Using set() on profile replaces the entire profile, risking data loss. push() is for adding new child nodes, not updating values.

Practice

1. Which Firebase method would you use to completely replace all data at a specific database path?

easy

A. push()

B. update()

C. set()

D. get()

Solution

1. Step 1: Understand set() behavior

   The set() method writes data to a specified path and replaces any existing data there.

2. Step 2: Compare with other methods

   update() modifies specific fields without replacing all data, push() adds new child nodes, and get() reads data.

3. Final Answer:

   set() -> Option C

4. Quick Check:

   Replace all data = set() [OK]

Hint: Use set() to overwrite all data at a path [OK]

Common Mistakes:

• Confusing update() with set() and expecting full replacement
• Using push() when needing to replace data
• Thinking get() writes data

2. Which of the following is the correct syntax to add a new child with a unique key under the path 'messages' using Firebase Realtime Database?

easy

A. firebase.database().ref('messages').set({ text: 'Hello' })

B. firebase.database().ref('messages').push({ text: 'Hello' })

C. firebase.database().ref('messages').update({ text: 'Hello' })

D. firebase.database().ref('messages').get({ text: 'Hello' })

Solution

1. Step 1: Identify method for unique child addition

   The push() method adds a new child with a unique key under the specified path.

2. Step 2: Check syntax correctness

   Using push({ text: 'Hello' }) under 'messages' correctly adds a new message with unique key.

3. Final Answer:

   firebase.database().ref('messages').push({ text: 'Hello' }) -> Option B

4. Quick Check:

   Unique child addition = push() [OK]

Hint: Use push() to add unique child nodes [OK]

Common Mistakes:

• Using set() which overwrites all children
• Using update() which only modifies existing fields
• Using get() which only reads data

3. Given the following Firebase code snippet:

firebase.database().ref('users/user1').update({ age: 30 });

What will happen to the existing data at 'users/user1' if it currently contains { name: 'Alice', age: 25 }?

medium

A. Only the 'age' field is updated to 30; 'name' remains 'Alice'

B. The entire 'users/user1' data is replaced with { age: 30 }

C. A new child with key 'age' is added under 'user1'

D. An error occurs because update() cannot be used here

Solution

1. Step 1: Understand update() behavior

   The update() method modifies only the specified fields without removing other existing data.

2. Step 2: Apply to given data

   Since 'age' is updated to 30, 'name' remains unchanged as 'Alice'.

3. Final Answer:

   Only the 'age' field is updated to 30; 'name' remains 'Alice' -> Option A

4. Quick Check:

   update() changes fields, keeps others [OK]

Hint: update() changes fields, keeps others intact [OK]

Common Mistakes:

• Thinking update() replaces all data like set()
• Confusing push() with update()
• Expecting an error when update() is valid

4. You want to update the user's email without removing other user data. Which Firebase method usage below contains an error?

medium

A. firebase.database().ref('users/user1').update({ email: '[e-mail]' })

B. firebase.database().ref('users/user1/email').set('[e-mail]')

C. firebase.database().ref('users/user1').update({ email: '[e-mail]', phone: '1234567890' })

D. firebase.database().ref('users/user1').push({ email: '[e-mail]' })

Solution

1. Step 1: Understand push() usage

   push() adds a new child with a unique key; it does not update existing fields.

2. Step 2: Analyze the goal

   To update email without removing other data, update() is correct; push() wrongly adds a new child instead.

3. Final Answer:

   Using push() to update email is incorrect -> Option D

4. Quick Check:

   push() adds new child, not update fields [OK]

Hint: Use update() to change fields, not push() [OK]

Common Mistakes:

• Using push() to update existing fields
• Using set() which overwrites all data
• Confusing update() and push() roles

5. You want to add multiple chat messages under 'chats' in Firebase Realtime Database, each with a unique key, and also update the user's last active timestamp without overwriting existing data. Which combination of methods should you use?

hard

A. Use push() for messages and update() for last active timestamp

B. Use push() for messages and set() for last active timestamp

C. Use update() for messages and set() for last active timestamp

D. Use set() for messages and update() for last active timestamp

Solution

1. Step 1: Add multiple messages with unique keys

   Use push() under 'chats' to add each message with a unique key without overwriting others.

2. Step 2: Update last active timestamp without overwriting

   Use update() on the user's data to change the timestamp without removing other fields.

3. Final Answer:

   Use push() for messages and update() for last active timestamp -> Option A

4. Quick Check:

   push() adds unique children, update() modifies fields [OK]

Hint: push() for new items, update() for field changes [OK]

Common Mistakes:

• Using set() for messages which overwrites all chats
• Using set() for timestamp which removes other user data
• Using update() to add unique children