$pull operator for removing from arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result after using $pull to remove a specific value?

Given a document { _id: 1, tags: ["red", "blue", "green", "blue"] }, what will be the tags array after applying {$pull: {tags: "blue"}}?

MongoDB

db.collection.updateOne({_id: 1}, {$pull: {tags: "blue"}})
db.collection.findOne({_id: 1})

A {"_id": 1, "tags": ["red", "green"]}

B {"_id": 1, "tags": ["red", "blue", "green"]}

C {"_id": 1, "tags": ["blue", "green"]}

D {"_id": 1, "tags": ["red", "green", "blue"]}

💡 Hint

The $pull operator removes all instances of the specified value from the array.

The $pull operator removes all elements equal to the specified value from the array. Here, all "blue" values are removed, leaving only "red" and "green".

❓ query_result

intermediate

Removing objects from an array using $pull with a condition

Consider a document { _id: 2, items: [{id: 1, qty: 5}, {id: 2, qty: 10}, {id: 3, qty: 3}] }. What will be the items array after applying {$pull: {items: {qty: {$lt: 5}}}}?

MongoDB

db.collection.updateOne({_id: 2}, {$pull: {items: {qty: {$lt: 5}}}})
db.collection.findOne({_id: 2})

A {"_id": 2, "items": []}

B {"_id": 2, "items": [{"id": 2, "qty": 10}]}

C {"_id": 2, "items": [{"id": 1, "qty": 5}, {"id": 2, "qty": 10}]}

D {"_id": 2, "items": [{"id": 1, "qty": 5}, {"id": 3, "qty": 3}]}

💡 Hint

The $pull condition removes all objects where qty is less than 5.

The $pull operator removes all array elements matching the condition {qty: {$lt: 5}}. Only the object with qty: 3 matches (3 < 5), so it is removed. The objects with qty: 5 and qty: 10 remain.

📝 Syntax

advanced

Identify the syntax error in this $pull update command

Which option contains a syntax error when trying to remove all elements equal to 5 from the array numbers?

MongoDB

db.collection.updateOne({_id: 3}, {$pull: {numbers: 5}})

A db.collection.updateOne({_id: 3}, {$pull: {numbers: 5}})

B db.collection.updateOne({_id: 3}, {$pull: {numbers: {ne: 5}}})

C db.collection.updateOne({_id: 3}, {$pull: {numbers: {$eq: 5}}})

D db.collection.updateOne({_id: 3}, {$pull: {numbers: {$in: [5]}}})

💡 Hint

Check the meaning and syntax of the $pull condition operators.

Option B contains a syntax error because it uses ne: 5 without the required $ prefix. Query operators in MongoDB must start with $ (e.g., $ne). Options A, B, and D are syntactically valid.

❓ query_result

advanced

What is the resulting array after $pull with multiple conditions?

Given a document { _id: 4, scores: [10, 20, 30, 40, 50] }, what will be the scores array after applying {$pull: {scores: {$gte: 20, $lte: 40}}}?

MongoDB

db.collection.updateOne({_id: 4}, {$pull: {scores: {$gte: 20, $lte: 40}}})
db.collection.findOne({_id: 4})

A {"_id": 4, "scores": [10, 50]}

B {"_id": 4, "scores": [10, 20, 30, 40, 50]}

C {"_id": 4, "scores": [20, 30, 40]}

D {"_id": 4, "scores": [10, 20, 50]}

💡 Hint

The $pull condition removes elements between 20 and 40 inclusive.

The $pull operator removes all elements where the value is greater than or equal to 20 and less than or equal to 40. So 20, 30, and 40 are removed, leaving 10 and 50.

🧠 Conceptual

expert

Why does $pull not remove elements when using an incorrect query condition?

You run {$pull: {items: {status: "active"}}} on a document where items is an array of strings like ["active", "inactive", "active"]. Why does $pull not remove any elements?

A Because $pull cannot remove duplicate elements.

B Because $pull only works on numeric arrays, not strings.

C Because $pull requires the use of $eq operator explicitly to match strings.

D Because $pull matches whole elements, and the condition expects objects but the array contains strings.

💡 Hint

Think about the data type of array elements and the query condition shape.

The $pull operator removes elements matching the condition. Here, the condition is an object {status: "active"}, but the array contains strings, not objects. So no elements match the condition, and nothing is removed.

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