Setting with merge option in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set data in Firestore with merge option.

Firebase

db.collection('users').doc('user1').set({ name: 'Alice' }, { [1]: true });

Drag options to blanks, or click blank then click option'

Aoverwrite

Bmerge

Cupdate

Dreplace

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'overwrite' will replace the entire document.

Using 'update' is a different method, not an option here.

The merge option allows you to update specific fields without overwriting the entire document.

2fill in blank

medium

Complete the code to merge new data into an existing Firestore document.

Firebase

db.collection('settings').doc('config').set({ theme: 'dark' }, { [1]: true });

Drag options to blanks, or click blank then click option'

Areplace

Boverwrite

Cmerge

Dupdate

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'update' method with 'merge' option.

Forgetting to pass the option as an object.

Using merge: true updates only the specified fields.

3fill in blank

hard

Fix the error in the Firestore set call to merge data correctly.

Firebase

db.collection('profiles').doc('user2').set({ age: 30 }, { [1] });

Drag options to blanks, or click blank then click option'

Atrue

Bmerge: false

Cmerge

Dmerge: true

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 'merge' without a value.

Passing 'true' without key.

The option must be an object with merge: true to merge fields.

4fill in blank

hard

Fill both blanks to set data with merge option and update the 'status' field.

Firebase

db.collection('orders').doc('order1').set({ status: 'shipped' }, { [1]: [2] });

Drag options to blanks, or click blank then click option'

Amerge

Btrue

Cfalse

Doverwrite

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'false' disables merging and overwrites data.

Using 'overwrite' is not a valid option.

Use merge: true to update only the 'status' field without overwriting the whole document.

5fill in blank

hard

Fill all three blanks to set nested data with merge option in Firestore.

Firebase

db.collection('users').doc('user3').set({ profile: { age: [1] } }, { [2]: [3] });

Drag options to blanks, or click blank then click option'

A25

Bmerge

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'false' disables merging and overwrites data.

Passing merge option incorrectly as a string.

Set the age to 25 and use merge: true to update nested fields without overwriting the whole document.

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