Setting with merge option in Firebase - Commands & Configuration

Introduction

When you update data in Firebase Firestore, sometimes you want to add or change only some parts without removing the rest. The merge option helps you update specific fields without deleting existing data.

When you want to add a new field to a document without removing existing fields.

When you want to update some fields in a document but keep other fields unchanged.

When you want to avoid overwriting the entire document accidentally.

When you want to combine new data with existing data safely.

When you want to update nested fields without replacing the whole nested object.

Commands

This command updates the document 'user123' in the 'users' collection by adding or updating the fields 'age' and 'city' without deleting other existing fields, thanks to the --merge flag.

Terminal

firebase firestore:documents:set users/user123 '{"age": 30, "city": "New York"}' --merge

Expected Output

✔  Document users/user123 updated with merge option.

→

--merge - Ensures only specified fields are updated, existing fields remain untouched.

This command retrieves the current data of the document 'user123' to verify that the merge update worked and other fields are still present.

Terminal

firebase firestore:documents:get users/user123

Expected Output

{

  "age": 30,

  "city": "New York",

  "name": "Alice",

  "email": "[e-mail]"

}

Key Concept

If you remember nothing else from this pattern, remember: the merge option updates only specified fields without deleting the rest of the document.

Common Mistakes

Updating a document without the --merge flag when you want to keep existing fields.

This overwrites the entire document, deleting fields not included in the update.

Always use --merge when you want to update specific fields without removing others.

Using incorrect JSON syntax in the update command.

The command fails because Firebase expects valid JSON for the data.

Use proper JSON format with double quotes around keys and string values.

Summary

Use the --merge flag to update only specific fields in a Firestore document.

Without --merge, updates overwrite the entire document, removing unspecified fields.

Verify updates by retrieving the document after applying changes.

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