Why updating documents matters in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update the age of a user to 30.

MongoDB

db.users.updateOne({name: "Alice"}, {$set: {age: [1])

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

A25

Bage

C"30"

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around 30 makes it a string, not a number.

Using the field name instead of the value.

The $set operator updates the field to the given value. Here, age is set to 30 as a number.

2fill in blank

medium

Complete the code to update all documents where status is 'pending' to have status 'complete'.

MongoDB

db.orders.updateMany({status: "pending"}, {$set: {status: [1])

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

A"complete"

B"pending"

Ccomplete

Dstatus

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string value.

Using the old status value instead of the new one.

The $set operator changes the status field to the string "complete" for all matching documents.

3fill in blank

hard

Fix the error in the update code to increment the score by 5.

MongoDB

db.players.updateOne({name: "Bob"}, {$inc: {score: [1])

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

A5

Bscore

C-5

D"5"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string "5" instead of number 5.

Using a negative number which decreases the score.

The $inc operator increments the score by the number 5. It must be a number, not a string.

4fill in blank

hard

Fill both blanks to update the user's city to 'Seattle' and add a new field 'verified' set to true.

MongoDB

db.users.updateOne({name: "Carol"}, {$set: {city: [1], verified: [2])

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

A"Seattle"

Btrue

Cfalse

D"Portland"

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true for verified.

Forgetting quotes around city name.

The city field is updated to the string "Seattle" and the verified field is added with the boolean value true.

5fill in blank

hard

Fill all three blanks to update the product price to 19.99, increase stock by 10, and set available to true.

MongoDB

db.products.updateOne({productId: 123}, {$set: {price: [1], available: [2], $inc: {stock: [3])

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

A19.99

Btrue

C10

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true for available.

Using a string instead of number for price or stock.

The price is set to 19.99, available is set to true, and stock is incremented by 10.

Practice

1. Why is it important to update documents in MongoDB instead of deleting and inserting new ones?

easy

A. Deleting and inserting is faster and safer.

B. Updating deletes the entire document automatically.

C. Updating keeps data consistent and avoids losing other fields.

D. MongoDB does not support updating documents.

Solution

1. Step 1: Understand document update purpose

   Updating modifies only specific fields, keeping other data intact.

2. Step 2: Compare update vs delete-insert

   Deleting and inserting risks losing data and is slower than updating.

3. Final Answer:

   Updating keeps data consistent and avoids losing other fields. -> Option C

4. Quick Check:

   Update preserves data = B [OK]

Hint: Update changes fields without losing data [OK]

Common Mistakes:

• Thinking delete-insert is faster
• Believing update removes whole document
• Assuming MongoDB can't update documents

2. Which of the following is the correct syntax to update the field age to 30 in a MongoDB document?

easy

A. db.collection.updateOne({name: 'John'}, {$set: {age: 30}})

B. db.collection.updateOne({name: 'John'}, {age: 30})

C. db.collection.update({name: 'John'}, {$change: {age: 30}})

D. db.collection.updateOne({name: 'John'}, {$update: {age: 30}})

Solution

1. Step 1: Identify correct update operator

   The $set operator updates specific fields without replacing the whole document.

2. Step 2: Check syntax correctness

   Only db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) uses updateOne with $set correctly.

3. Final Answer:

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) -> Option A

4. Quick Check:

   Use $set to update fields = C [OK]

Hint: Use $set inside updateOne to change fields [OK]

Common Mistakes:

• Omitting $set operator
• Using wrong operator like $change or $update
• Passing field directly without $set

3. Given the collection documents:

{"name": "Alice", "score": 50}

What will be the result after running:

db.collection.updateOne({name: "Alice"}, {$set: {score: 75}});
db.collection.find({name: "Alice"}).toArray();

medium

A. []

B. [{"name": "Alice", "score": 75}]

C. [{"name": "Alice"}]

D. [{"name": "Alice", "score": 50}]

Solution

1. Step 1: Understand updateOne with $set

   The command changes the score field from 50 to 75 for the document where name is "Alice".

2. Step 2: Check find query result

   The find query returns the updated document with score now 75.

3. Final Answer:

   [{"name": "Alice", "score": 75}] -> Option B

4. Quick Check:

   Update changes score to 75 = D [OK]

Hint: Update changes field value, find shows updated document [OK]

Common Mistakes:

• Expecting old score after update
• Thinking update removes other fields
• Assuming update adds new document

4. What is wrong with this update command?

db.collection.updateOne({name: "Bob"}, {score: 100});

medium

A. Missing $set operator, so it replaces the whole document.

B. The filter query is incorrect syntax.

C. updateOne cannot update numeric fields.

D. The collection name is invalid.

Solution

1. Step 1: Check update command structure

   The update document must use an operator like $set to update fields without replacing the whole document.

2. Step 2: Understand effect of missing $set

   Without $set, the document is replaced entirely with {score: 100}, losing other fields.

3. Final Answer:

   Missing $set operator, so it replaces the whole document. -> Option A

4. Quick Check:

   Always use $set to update fields [OK]

Hint: Always include $set to update fields safely [OK]

Common Mistakes:

• Omitting $set and replacing document
• Thinking updateOne syntax is wrong
• Believing updateOne can't update numbers

5. You want to update the status field to "active" only if it currently exists in the document. Which update command achieves this safely without creating new fields?

hard

A. db.collection.updateMany({status: {$exists: false}}, {$set: {status: "active"}})

B. db.collection.updateMany({}, {$set: {status: "active"}})

C. db.collection.updateMany({status: null}, {$set: {status: "active"}})

D. db.collection.updateMany({status: {$exists: true}}, {$set: {status: "active"}})

Solution

1. Step 1: Use filter to check field existence

   The filter {status: {$exists: true}} selects documents where status field exists.

2. Step 2: Update only matching documents

   The $set updates status to "active" only for those documents, avoiding creating new fields.

3. Final Answer:

   db.collection.updateMany({status: {$exists: true}}, {$set: {status: "active"}}) -> Option D

4. Quick Check:

   Filter with $exists true to update safely = A [OK]

Hint: Filter with $exists:true to update only existing fields [OK]

Common Mistakes:

• Updating all documents regardless of field existence
• Using $exists:false which matches missing fields
• Filtering with null instead of $exists