Setting with merge option in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the 'merge' option do when setting data in Firebase Firestore?

The 'merge' option updates only the specified fields in a document without deleting other existing fields. It merges new data with existing data.

beginner

How do you use the 'merge' option in Firestore's set() method?

You pass an object with { merge: true } as the second argument to the set() method, like: set(data, { merge: true }).

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What happens if you set data without the merge option in Firestore?

The entire document is overwritten, and any fields not included in the new data are deleted.

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Why is using the merge option useful in real-life applications?

It lets you update only parts of a document, like changing a user's email without losing their profile info, similar to editing one part of a form without erasing the rest.

beginner

Can the merge option be used to add new fields to an existing Firestore document?

Yes, merge allows adding new fields while keeping existing fields intact.

What does setting { merge: true } do in Firestore's set() method?

A Deletes the entire document

B Prevents any changes to the document

C Creates a new collection

D Updates only specified fields without deleting others

The merge option updates only the fields you specify, keeping other fields unchanged.

If you call set() without merge on an existing document, what happens?

A Document is overwritten completely

B Only specified fields update

C Document is deleted

D Nothing changes

Without merge, set() replaces the entire document with new data.

Which of these is a benefit of using merge when updating Firestore documents?

A It creates a backup copy

B It deletes old data automatically

C It merges new data with existing data

D It locks the document from changes

Merge combines new data with existing fields without deleting them.

How do you pass the merge option in Firestore's set() method?

A set(data, { merge: true })

B set(data, { overwrite: false })

C set(data, true)

D set(data, { update: true })

The merge option is passed as an object: { merge: true }.

Can merge option add new fields to an existing document?

A No, it only updates existing fields

B Yes, it adds new fields without removing old ones

C No, it deletes all fields first

D Yes, but only if the document is empty

Merge allows adding new fields while keeping existing fields intact.

Explain how the merge option affects updating a Firestore document.

Think about how you can change part of a document without losing the rest.

Describe a real-life scenario where using the merge option is helpful.

Imagine changing your email address but keeping your name and photo unchanged.

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