$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.