Writing data (set, update, push) in Firebase - Commands & Configuration

Introduction

When you want to save or change information in a Firebase database, you use writing commands. These commands let you add new data, change existing data, or add data with a unique ID automatically.

When you want to save a new user's profile information in your app.

When you need to change the status of an order in your database.

When you want to add a new message to a chat without overwriting others.

When you want to update only part of a user's settings without changing the rest.

When you want to add data with a unique key generated automatically.

Commands

This command sets the data at the path /users/user1 to the given JSON object. It creates or replaces the data at that location.

Terminal

firebase database:set /users/user1 '{"name":"Alice","age":30}'

Expected Output

✔  Data at /users/user1 set successfully.

This command updates only the age field of the user1 data without changing other fields like name.

Terminal

firebase database:update /users/user1 '{"age":31}'

Expected Output

✔  Data at /users/user1 updated successfully.

This command adds a new message under /messages with a unique key generated automatically, so it does not overwrite existing messages.

Terminal

firebase database:push /messages '{"text":"Hello!","sender":"user1"}'

Expected Output

✔  Data pushed to /messages with key -Mxyz123abc.

This command retrieves the current data at /users/user1 to verify the changes made by set or update.

Terminal

firebase database:get /users/user1

Expected Output

{

  "name": "Alice",

  "age": 31

}

Key Concept

If you remember nothing else from this pattern, remember: set replaces data, update changes parts of data, and push adds new data with a unique key.

Common Mistakes

Using set when you want to add data without deleting existing data.

Set replaces all data at the path, so existing data will be lost.

Use update to change parts of data or push to add new entries without overwriting.

Using push when you want to update existing data at a known path.

Push always creates a new unique key, so it won't update existing data.

Use update or set with the exact path to modify existing data.

Summary

Use 'set' to create or replace data at a specific path.

Use 'update' to change only certain fields without removing others.

Use 'push' to add new data with a unique key automatically.

Verify changes by retrieving data with 'get' command.

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