Setting with merge option in Firebase - Mini Project: Build & Apply

Firebase Firestore: Setting Document Data with Merge Option

📖 Scenario: You are building a simple app that stores user profiles in Firebase Firestore. Sometimes you want to update only part of a user's profile without overwriting the entire document.

🎯 Goal: Learn how to use the set method with the merge: true option to update parts of a Firestore document safely.

📋 What You'll Learn

Create a Firestore document reference for a user with ID 'user123'.

Create an initial user profile object with name and age.

Use set to save the initial profile to Firestore.

Create an update object with a new age and city.

Use set with { merge: true } to update the user profile without deleting existing fields.

💡 Why This Matters

🌍 Real World

Updating user profiles or settings in Firestore without deleting existing data is common in apps.

💼 Career

Understanding how to safely update Firestore documents is essential for backend and full-stack developers working with Firebase.

1

Create Firestore document reference and initial user profile

Create a variable called userRef that references the Firestore document at users/user123. Then create a variable called initialProfile with the exact object { name: 'Alice', age: 30 }.

Firebase

// Your code here

Hint

Use firestore.collection('users').doc('user123') to get the document reference.

2

Save the initial profile to Firestore

Use userRef.set(initialProfile) to save the initial profile object to Firestore.

Firebase

const userRef = firestore.collection('users').doc('user123');
const initialProfile = { name: 'Alice', age: 30 };
// Your code here

Hint

Call set on userRef with initialProfile as argument.

3

Create an update object with new age and city

Create a variable called updateProfile with the exact object { age: 31, city: 'New York' }.

Firebase

const userRef = firestore.collection('users').doc('user123');
const initialProfile = { name: 'Alice', age: 30 };
userRef.set(initialProfile);
// Your code here

Hint

Define updateProfile with the new age and city fields.

4

Update the user profile using set with merge option

Use userRef.set(updateProfile, { merge: true }) to update the Firestore document without overwriting the entire profile.

Firebase

const userRef = firestore.collection('users').doc('user123');
const initialProfile = { name: 'Alice', age: 30 };
userRef.set(initialProfile);
const updateProfile = { age: 31, city: 'New York' };
// Your code here

Hint

Call set on userRef with updateProfile and the option { merge: true }.

Practice

1. What does the merge: true option do when updating a Firestore document?

easy

A. It deletes the entire document before updating.

B. It updates only the specified fields without deleting others.

C. It creates a new document with the same ID.

D. It prevents any updates to the document.

Solution

1. Step 1: Understand Firestore update behavior

   By default, setting data replaces the whole document, removing unspecified fields.

2. Step 2: Role of merge: true

   Using merge: true updates only the fields you specify, keeping other fields intact.

3. Final Answer:

   It updates only the specified fields without deleting others. -> Option B

4. Quick Check:

   Merge option = partial update [OK]

Hint: Merge means update fields, keep others safe [OK]

Common Mistakes:

• Thinking merge deletes the whole document
• Confusing merge with creating new documents
• Assuming merge prevents updates

2. Which of the following is the correct syntax to update a Firestore document with merge option in JavaScript?

easy

A. docRef.set({name: 'Alice'}, merge=true);

B. docRef.update({name: 'Alice'}, {merge: true});

C. docRef.set({name: 'Alice'}, {merge: true});

D. docRef.update({name: 'Alice'}, merge=true);

Solution

1. Step 1: Identify correct method and options

   Firestore's set method accepts an options object as second parameter for merge.

2. Step 2: Check syntax correctness

   The correct syntax is set(data, {merge: true}). The update method does not accept merge option.

3. Final Answer:

   docRef.set({name: 'Alice'}, {merge: true}); -> Option C

4. Quick Check:

   set with {merge: true} is correct syntax [OK]

Hint: Use set() with {merge: true}, not update() [OK]

Common Mistakes:

• Using update() with merge option
• Passing merge as a separate argument without braces
• Using merge=true instead of {merge: true}

3. Given the Firestore document data: {name: 'Bob', age: 30, city: 'NY'}, what will be the document content after running docRef.set({age: 31}, {merge: true});?

medium

A. {name: 'Bob', age: 30, city: 'NY', age: 31}

B. {age: 31}

C. {name: 'Bob', city: 'NY'}

D. {name: 'Bob', age: 31, city: 'NY'}

Solution

1. Step 1: Understand existing document

   The document has fields: name='Bob', age=30, city='NY'.

2. Step 2: Effect of set with merge

   Using set with {merge: true} updates only the specified field age to 31, keeping others unchanged.

3. Final Answer:

   {name: 'Bob', age: 31, city: 'NY'} -> Option D

4. Quick Check:

   Merge updates age, keeps name and city [OK]

Hint: Merge updates given fields, keeps rest intact [OK]

Common Mistakes:

• Thinking merge replaces whole document
• Expecting duplicate keys in document
• Assuming fields not mentioned get deleted

4. You try to update a Firestore document with docRef.set({score: 100}, merge=true); but it throws an error. What is the problem?

medium

A. The merge option must be passed as an object: {merge: true}.

B. The set method does not support merge option.

C. The score field cannot be updated.

D. You must use update() instead of set() for merge.

Solution

1. Step 1: Check merge option syntax

   The merge option must be passed as an object: {merge: true}, not as merge=true.

2. Step 2: Confirm set supports merge

   The set method supports merge option correctly when passed as an object.

3. Final Answer:

   The merge option must be passed as an object: {merge: true}. -> Option A

4. Quick Check:

   Merge option syntax requires braces [OK]

Hint: Pass merge as {merge: true}, not merge=true [OK]

Common Mistakes:

• Passing merge as a boolean without braces
• Using update() with merge option
• Assuming merge disables updates

5. You have a Firestore document {name: 'Eve', stats: {wins: 5, losses: 2}}. You want to update only wins to 6 without removing losses. Which code correctly updates the document using merge?

hard

A. docRef.set({stats: {wins: 6}}, {mergeFields: ['stats.wins']});

B. docRef.set({stats: {wins: 6, losses: 2}}, {merge: true});

C. docRef.set({stats: {wins: 6}}, {merge: true});

D. docRef.update({stats: {wins: 6}});

Solution

1. Step 1: Understand nested field update with merge

   Using {merge: true} with nested objects replaces the whole nested object, removing unspecified keys.

2. Step 2: Use mergeFields to update nested field only

   Passing {mergeFields: ['stats.wins']} updates only the nested wins field without deleting losses.

3. Final Answer:

   docRef.set({stats: {wins: 6}}, {mergeFields: ['stats.wins']}); -> Option A

4. Quick Check:

   mergeFields updates nested fields precisely [OK]

Hint: Use mergeFields to update nested fields safely [OK]

Common Mistakes:

• Using merge true replaces whole nested object
• Not specifying mergeFields for nested updates
• Assuming update() always merges nested fields