Setting with merge option in Firebase - Time & Space Complexity

Time Complexity: Setting with merge option

O(n)

Understanding Time Complexity

When we update data in Firebase using the merge option, it changes only parts of the data instead of replacing everything.

We want to know how the time it takes grows as the data size grows.

Scenario Under Consideration

Analyze the time complexity of this Firebase update with merge.

const docRef = firestore.collection('users').doc('user123');
const updateData = { age: 30, city: 'New York' };

await docRef.set(updateData, { merge: true });

This updates only the fields given, keeping other fields unchanged in the document.

Identify Repeating Operations

Look at what happens when we run this update.

• Primary operation: One network call to update the document with merge.
• How many times: Exactly once per update call.

How Execution Grows With Input

As the number of fields in updateData grows, the update sends more data but still only one call.

Input Size (fields to update)	Approx. API Calls/Operations
10	1 network call with 10 fields
100	1 network call with 100 fields
1000	1 network call with 1000 fields

Pattern observation: The number of calls stays the same, but the data size sent grows with the number of fields.

Final Time Complexity

Time Complexity: O(n)

This means the time grows linearly with the number of fields you update in the merge.

Common Mistake

[X] Wrong: "Using merge means the update time is always constant no matter how many fields I update."

[OK] Correct: Even though there is only one update call, the amount of data sent grows with the number of fields, so time grows too.

Interview Connect

Understanding how partial updates scale helps you design efficient data updates and shows you know how cloud calls behave as data grows.

Self-Check

"What if we replaced the merge option with a full set without merge? How would the time complexity change?"

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