$set operator for setting fields in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the field "age" to 30 in the update operation.

MongoDB

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

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

A25

B30

C35

D40

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number for the age value.

Forgetting to use the $set operator.

The $set operator updates the value of the specified field. Here, we set age to 30.

2fill in blank

medium

Complete the code to set the "status" field to "active" for all matching documents.

MongoDB

db.users.updateMany({ subscribed: true }, { $set: { status: [1] } })

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

Afalse

Btrue

C"inactive"

D"active"

Attempts:

3 left

💡 Hint

Common Mistakes

Using a boolean value instead of a string.

Omitting the quotes around the string value.

The $set operator updates the status field to the string "active" for all users who are subscribed.

3fill in blank

hard

Fix the error in the update command to correctly set the "score" field to 100.

MongoDB

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

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

A{ score: 100 }

B[ score: 100 ]

Cscore: 100

D"score: 100"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using curly braces around the field and value.

Using square brackets or quotes incorrectly.

The $set operator requires an object with field-value pairs. So, { score: 100 } is correct.

4fill in blank

hard

Fill both blanks to set the "level" field to 5 and "active" field to true in one update.

MongoDB

db.accounts.updateOne({ user: "Eve" }, { $set: { [1]: [2], active: true } })

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

Alevel

B5

Cactive

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the value before the field name.

Using quotes incorrectly around numbers.

To set multiple fields, you list them inside the $set object. Here, the first blank is the field name level, and the second blank is its value 5.

5fill in blank

hard

Fill all three blanks to set "score" to 90, "level" to 3, and "active" to false in one update.

MongoDB

db.players.updateOne({ id: 123 }, { $set: { [1]: [2], [3]: 3, active: false } })

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

Ascore

B90

Clevel

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up field names and values.

Forgetting commas between fields.

The $set operator updates multiple fields by listing them inside the object. Here, score is set to 90, level is set to 3, and active is set to false.

Practice

1. What does the $set operator do in MongoDB?

easy

A. It updates the value of specified fields or adds them if they don't exist.

B. It deletes specified fields from a document.

C. It replaces the entire document with a new one.

D. It creates a new collection in the database.

Solution

1. Step 1: Understand the purpose of $set

   The $set operator is used to update existing fields or add new fields in a MongoDB document without affecting other fields.

2. Step 2: Compare with other operations

   Deleting fields uses $unset, replacing documents uses replaceOne, and creating collections is unrelated to $set.

3. Final Answer:

   It updates the value of specified fields or adds them if they don't exist. -> Option A

4. Quick Check:

   $set updates or adds fields [OK]

Hint: Remember: $set changes or adds fields only [OK]

Common Mistakes:

• Confusing $set with $unset which deletes fields
• Thinking $set replaces the whole document
• Assuming $set creates collections

2. Which of the following is the correct syntax to set the field age to 30 in a document using $set?

easy

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

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

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

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

Solution

1. Step 1: Identify correct $set syntax

   The $set operator requires an object with field-value pairs inside curly braces: {$set: {field: value}}.

2. Step 2: Check each option

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}) uses correct syntax with {age: 30}. Options A, B, and C use invalid JavaScript or MongoDB syntax.

3. Final Answer:

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

4. Quick Check:

   Correct $set syntax uses object with field-value pairs [OK]

Hint: Use {$set: {field: value}} with curly braces [OK]

Common Mistakes:

• Using assignment (=) inside $set
• Using quotes incorrectly around field names
• Using array brackets instead of object braces

3. Given the document { name: 'Alice', city: 'Paris' }, what will be the document after running db.users.updateOne({name: 'Alice'}, {$set: {city: 'London', age: 25}})?

medium

A. { city: 'London', age: 25 }

B. { name: 'Alice', city: 'Paris', age: 25 }

C. { name: 'Alice', city: 'London', age: 25 }

D. { name: 'Alice', city: 'London' }

Solution

1. Step 1: Understand $set effect on fields

   The $set operator updates existing fields and adds new fields without removing others.

2. Step 2: Apply update to the document

   Field city changes from 'Paris' to 'London', and new field age with value 25 is added. The name field remains unchanged.

3. Final Answer:

   { name: 'Alice', city: 'London', age: 25 } -> Option C

4. Quick Check:

   $set updates and adds fields, keeps others [OK]

Hint: Only fields in $set change; others stay same [OK]

Common Mistakes:

• Assuming $set removes fields not listed
• Thinking $set only adds but does not update
• Confusing updateOne with replaceOne

4. You run this update: db.products.updateOne({id: 101}, {$set: {price: 20, stock}}). What is the problem?

medium

A. Missing value for the field 'stock' inside $set causes a syntax error.

B. The query filter {id: 101} is invalid.

C. You cannot update multiple fields with $set.

D. The update will delete the document.

Solution

1. Step 1: Analyze the $set object

   The $set operator requires each field to have a value. Here, stock is listed without a value, which is invalid syntax.

2. Step 2: Check other parts of the update

   The filter {id: 101} is valid, and $set can update multiple fields. The update does not delete documents.

3. Final Answer:

   Missing value for the field 'stock' inside $set causes a syntax error. -> Option A

4. Quick Check:

   Each field in $set must have a value [OK]

Hint: Every field in $set needs a value [OK]

Common Mistakes:

• Leaving out values for fields in $set
• Confusing filter syntax with update syntax
• Thinking $set deletes documents

5. You want to update all documents in the employees collection to add a new field status with value 'active' only if the field status does not already exist. Which update command using $set is correct?

hard

A. db.employees.updateMany({}, {$set: {status: 'active'}})

B. db.employees.updateMany({status: {$ne: 'active'}}, {$set: {status: 'active'}})

C. db.employees.updateMany({status: null}, {$set: {status: 'active'}})

D. db.employees.updateMany({status: {$exists: false}}, {$set: {status: 'active'}})

Solution

1. Step 1: Identify condition to update only missing 'status'

   The filter {status: {$exists: false}} selects documents where the 'status' field does not exist.

2. Step 2: Use $set to add 'status' field

   The update {$set: {status: 'active'}} adds the 'status' field with value 'active' only to those documents.

3. Step 3: Check other options

   db.employees.updateMany({}, {$set: {status: 'active'}}) updates all documents regardless of existing 'status'. db.employees.updateMany({status: null}, {$set: {status: 'active'}}) only matches documents with 'status' equal to null, not missing. db.employees.updateMany({status: {$ne: 'active'}}, {$set: {status: 'active'}}) updates documents where 'status' is not 'active', which may overwrite existing values.

4. Final Answer:

   db.employees.updateMany({status: {$exists: false}}, {$set: {status: 'active'}}) -> Option D

5. Quick Check:

   Use {$exists: false} to target missing fields [OK]

Hint: Filter with {$exists: false} to add missing fields only [OK]

Common Mistakes:

• Updating all documents without filter
• Using null instead of $exists for missing fields
• Overwriting existing 'status' values unintentionally