deleteMany method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output count after deleteMany?

Given a collection users with documents:

{"name": "Alice", "age": 25}, {"name": "Bob", "age": 30}, {"name": "Carol", "age": 25}

What will be the deletedCount after running:

db.users.deleteMany({ age: 25 })

MongoDB

db.users.deleteMany({ age: 25 })

A 1

B 2

C 3

D 0

💡 Hint

Count how many documents have age 25.

Two documents have age 25: Alice and Carol. So deleteMany removes 2 documents.

📝 Syntax

intermediate

Which deleteMany syntax is correct?

Which of the following deleteMany method calls is syntactically correct in MongoDB?

A db.collection.deleteMany({ age: { $gt: 20 } })

B db.collection.deleteMany(age > 20)

C db.collection.deleteMany({ age: gt 20 })

D db.collection.deleteMany({ age: > 20 })

💡 Hint

MongoDB uses query objects with operators starting with $.

Option A uses the correct MongoDB query syntax with $gt operator inside an object.

❓ optimization

advanced

Optimizing deleteMany for large collections

You want to delete all documents where status is "inactive" in a large collection. Which approach is best for performance?

A db.collection.deleteMany({ status: "inactive" })

B db.collection.find({ status: "inactive" }).forEach(doc => db.collection.deleteOne({ _id: doc._id }))

C db.collection.deleteMany({ status: { $eq: "inactive" } })

D db.collection.deleteMany({ status: { $ne: "active" } })

💡 Hint

Consider the efficiency of bulk delete vs looping deletes.

Option A uses a single bulk deleteMany operation which is faster than deleting documents one by one.

🧠 Conceptual

advanced

Effect of deleteMany with empty filter

What happens if you run db.collection.deleteMany({}) on a collection?

A Deletes documents with null fields only

B Deletes no documents

C Deletes all documents in the collection

D Throws a syntax error

💡 Hint

An empty filter matches all documents.

An empty filter {} matches every document, so deleteMany deletes all documents.

🔧 Debug

expert

Why does this deleteMany not delete any documents?

Given documents:

{"name": "Dave", "age": 40}, {"name": "Eve", "age": 35}

Why does this command delete zero documents?

db.users.deleteMany({ age: { $lt: 30 } })

A The collection name is incorrect

B Syntax error in the query

C deleteMany only deletes one document

D No documents have age less than 30

💡 Hint

Check the ages of documents compared to the filter.

Both documents have age 35 or 40, none less than 30, so no documents match and none are deleted.