$pull operator for removing from arrays in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to remove all elements equal to 5 from the 'scores' array.

MongoDB

db.students.updateOne({name: 'Alice'}, {$pull: {scores: [1])

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

A"5"

B5

C{ $eq: 5 }

D[5]

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around numbers removes strings, not numbers.

Passing an array instead of a value causes errors.

The $pull operator removes all instances of the specified value from the array. Here, 5 is the value to remove.

2fill in blank

medium

Complete the code to remove all elements greater than 10 from the 'grades' array.

MongoDB

db.courses.updateMany({}, {$pull: {grades: { [1]: 10 }}})

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

A$eq

B$lt

C$gt

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt removes elements less than 10, which is wrong here.

Using $eq removes only elements equal to 10.

The $gt operator means 'greater than'. It removes all elements greater than 10.

3fill in blank

hard

Fix the error in the code to remove all elements less than or equal to 3 from the 'values' array.

MongoDB

db.data.updateOne({}, {$pull: {values: { [1]: 3 }}})

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

A$gte

B$eq

C$lt

D$lte

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt excludes elements equal to 3.

Using $gte removes elements greater than or equal to 3, which is opposite.

The $lte operator means 'less than or equal to', which matches the requirement.

4fill in blank

hard

Fill both blanks to remove all elements that are strings equal to 'remove' from the 'tags' array.

MongoDB

db.collection.updateMany({}, {$pull: {tags: { [1]: [2] }}})

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

A$eq

B"remove"

C'remove'

D$ne

Attempts:

3 left

💡 Hint

Common Mistakes

Using $ne removes elements not equal to the value, which is wrong.

Using single quotes may cause syntax issues in some environments.

The $eq operator matches elements equal to the given value. The value must be a string, so quotes are needed.

5fill in blank

hard

Fill both blanks to remove all elements from the 'items' array where the 'price' field is less than 20.

MongoDB

db.store.updateOne({id: 1}, {$pull: {items: { [1]: { [2]: 20 }}}})

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

Aprice

B$lt

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Using cost instead of price as field name.

Using $gt instead of $lt operator.

The field to check is price. The operator for 'less than' is $lt. The query removes items with price less than 20.

Practice

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

easy

A. Removes all array elements that match a specified condition.

B. Adds new elements to an array.

C. Replaces the entire array with a new one.

D. Sorts the elements inside an array.

Solution

1. Step 1: Understand the purpose of $pull

   The $pull operator is used to remove elements from an array that match a given condition.

2. Step 2: Compare with other array operators

   Unlike $push which adds elements, $pull removes matching elements without affecting others.

3. Final Answer:

   Removes all array elements that match a specified condition. -> Option A

4. Quick Check:

   $pull = Remove matching elements [OK]

Hint: Remember: $pull always removes matching array items [OK]

Common Mistakes:

• Confusing $pull with $push (which adds elements)
• Thinking $pull replaces the whole array
• Assuming $pull sorts or modifies elements

2. Which of the following is the correct syntax to remove the value 5 from the array field numbers in MongoDB?

easy

A. db.collection.updateOne({}, {$pull: {numbers: 5}})

B. db.collection.updateOne({}, {$pull: {numbers: {$ne: 5}}})

C. db.collection.updateOne({}, {$remove: {numbers: 5}})

D. db.collection.updateOne({}, {$pop: {numbers: 5}})

Solution

1. Step 1: Identify the correct operator and syntax

   The $pull operator removes elements matching the value directly, so {$pull: {numbers: 5}} is correct.

2. Step 2: Check other options for errors

   db.collection.updateOne({}, {$pop: {numbers: 5}}) uses $pop which only removes first/last elements. db.collection.updateOne({}, {$pull: {numbers: {$ne: 5}}}) uses {$ne: 5} inside $pull which removes non-matching elements. db.collection.updateOne({}, {$remove: {numbers: 5}}) uses invalid $remove operator.

3. Final Answer:

   db.collection.updateOne({}, {$pull: {numbers: 5}}) -> Option A

4. Quick Check:

   Simple value removal uses $pull: {field: value} [OK]

Hint: Use $pull: {field: value} to remove simple values [OK]

Common Mistakes:

• Using $remove instead of $pull
• Using $pop which removes first or last element only
• Adding unnecessary query operators for simple values

3. Given the document { _id: 1, tags: ["red", "blue", "green", "blue"] }, what will be the tags array after running db.collection.updateOne({_id: 1}, {$pull: {tags: "blue"}})?

medium

A. ["blue", "blue"]

B. ["red", "blue", "green"]

C. ["red", "green", "blue"]

D. ["red", "green"]

Solution

1. Step 1: Understand $pull removes all matching elements

   The $pull operator removes every element equal to "blue" from the array.

2. Step 2: Remove all "blue" elements from the array

   Original array: ["red", "blue", "green", "blue"]. After removal: ["red", "green"] because both "blue" elements are removed.

3. Final Answer:

   ["red", "green"] -> Option D

4. Quick Check:

   All "blue" removed = ["red", "green"] [OK]

Hint: Remember $pull removes all matching elements, not just one [OK]

Common Mistakes:

• Removing only the first matching element
• Leaving one "blue" element by mistake
• Confusing $pull with $pop or $pullAll

4. You want to remove all numbers less than 10 from the array field scores. Which of the following update commands will NOT work correctly?

medium

A. db.collection.updateOne({}, {$pull: {scores: { $lt: 10 }}})

B. db.collection.updateOne({}, {$pull: {scores: {$lt: 10}}})

C. db.collection.updateOne({}, {$pull: {scores: { $gte: 10 }}})

D. db.collection.updateOne({}, {$pull: {scores: {$lt:10}}})

Solution

1. Step 1: Understand the condition to remove numbers less than 10

   The correct condition is {$lt: 10} to remove numbers less than 10.

2. Step 2: Analyze each option

   Options B, C, and D use {$lt: 10} correctly. db.collection.updateOne({}, {$pull: {scores: { $gte: 10 }}}) uses {$gte: 10}, which removes numbers greater than or equal to 10, the opposite of the goal.

3. Final Answer:

   db.collection.updateOne({}, {$pull: {scores: { $gte: 10 }}}) -> Option C

4. Quick Check:

   Use $lt to remove less than 10, not $gte [OK]

Hint: Use correct comparison operator inside $pull condition [OK]

Common Mistakes:

• Using wrong comparison operator ($gte instead of $lt)
• Confusing $pull condition syntax
• Repeating same option without change (typo)

5. Consider documents with a field items containing objects like {name: "apple", qty: 5}. How would you remove all items where qty is 0 using $pull?

hard

A. db.collection.updateMany({}, {$pull: {items: {qty: {$ne: 0}}}})

B. db.collection.updateMany({}, {$pull: {items: {qty: 0}}})

C. db.collection.updateMany({}, {$pull: {items: {name: "apple"}}})

D. db.collection.updateMany({}, {$pull: {items: {qty: {$gt: 0}}}})

Solution

1. Step 1: Identify the condition to remove items with qty 0

   We want to remove array elements where the field qty equals 0.

2. Step 2: Use $pull with a query object matching qty: 0

   The correct syntax is {$pull: {items: {qty: 0}}} which removes all objects with qty 0.

3. Final Answer:

   db.collection.updateMany({}, {$pull: {items: {qty: 0}}}) -> Option B

4. Quick Check:

   Match exact condition inside $pull to remove objects [OK]

Hint: Use $pull with object condition to remove matching objects [OK]

Common Mistakes:

• Removing by wrong field (like name instead of qty)
• Using $ne or $gt incorrectly inside $pull
• Confusing $pull with $push or $pop