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.

Practice

1. What does the deleteMany method do in MongoDB?

easy

A. Inserts multiple documents into a collection.

B. Deletes only one document regardless of the filter.

C. Removes all documents that match a given filter.

D. Updates multiple documents based on a filter.

Solution

1. Step 1: Understand the purpose of deleteMany

   The deleteMany method is designed to remove multiple documents that match a specified filter in a collection.

2. Step 2: Compare with other methods

   Unlike deleteOne which deletes a single document, deleteMany deletes all matching documents. It does not update or insert documents.

3. Final Answer:

   Removes all documents that match a given filter. -> Option C

4. Quick Check:

   deleteMany removes multiple matching documents = D [OK]

Hint: deleteMany deletes all matching documents, not just one [OK]

Common Mistakes:

• Confusing deleteMany with deleteOne
• Thinking deleteMany updates documents
• Assuming deleteMany inserts documents

2. Which of the following is the correct syntax to delete all documents where status is "inactive" using deleteMany?

easy

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

B. db.collection.deleteMany("status = 'inactive'")

C. db.collection.deleteMany(status == "inactive")

D. db.collection.deleteMany([status: "inactive"])

Solution

1. Step 1: Identify correct filter syntax

   The filter in deleteMany must be a JSON object with key-value pairs, like {status: "inactive"}.

2. Step 2: Check each option

   db.collection.deleteMany({status: "inactive"}) uses correct JSON object syntax. Options B, C, and D use invalid syntax for MongoDB filters.

3. Final Answer:

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

4. Quick Check:

   Filter must be JSON object = A [OK]

Hint: Use JSON object for filter in deleteMany [OK]

Common Mistakes:

• Using string instead of object for filter
• Using comparison operators inside filter incorrectly
• Using array syntax for filter

3. Given the collection users with documents:

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

What will be the result of db.users.deleteMany({age: 25})?

medium

A. Deletes 2 documents where age is 25

B. Deletes 1 document where age is 25

C. Deletes all documents regardless of age

D. No documents deleted

Solution

1. Step 1: Identify matching documents

   Documents with age: 25 are Alice and Carol, so 2 documents match the filter.

2. Step 2: Understand deleteMany behavior

   deleteMany removes all documents matching the filter, so both Alice and Carol will be deleted.

3. Final Answer:

   Deletes 2 documents where age is 25 -> Option A

4. Quick Check:

   deleteMany removes all matching documents = A [OK]

Hint: deleteMany removes all matching, not just one [OK]

Common Mistakes:

• Thinking only one document is deleted
• Assuming deleteMany deletes all documents
• Confusing filter criteria

4. What is wrong with this code snippet?

db.products.deleteMany("{category: 'electronics'}")

medium

A. deleteMany cannot delete documents by category.

B. The collection name is incorrect.

C. Missing semicolon at the end.

D. Filter should be an object, not a string.

Solution

1. Step 1: Check filter argument type

   The filter argument must be a JSON object, but here it is passed as a string.

2. Step 2: Validate other parts

   deleteMany can delete by any filter, semicolon is optional in JS, and collection name is valid.

3. Final Answer:

   Filter should be an object, not a string. -> Option D

4. Quick Check:

   Filter must be object, not string = B [OK]

Hint: Pass filter as object, not string, in deleteMany [OK]

Common Mistakes:

• Passing filter as string instead of object
• Assuming semicolon is mandatory
• Misnaming collection

5. You want to delete all documents from the orders collection where the status is either "cancelled" or "returned". Which deleteMany filter correctly achieves this?

hard

A. db.orders.deleteMany({status: "cancelled" || "returned"})

B. db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}})

C. db.orders.deleteMany({status: {$or: ["cancelled", "returned"]}})

D. db.orders.deleteMany({status: ["cancelled", "returned"]})

Solution

1. Step 1: Understand filter for multiple values

   To match documents where a field equals any value in a list, MongoDB uses the $in operator with an array of values.

2. Step 2: Evaluate each option

   db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}}) correctly uses $in. db.orders.deleteMany({status: "cancelled" || "returned"}) uses JavaScript OR incorrectly inside an object. db.orders.deleteMany({status: {$or: ["cancelled", "returned"]}}) misuses $or inside a field. db.orders.deleteMany({status: ["cancelled", "returned"]}) uses an array directly, which is invalid.

3. Final Answer:

   db.orders.deleteMany({status: {$in: ["cancelled", "returned"]}}) -> Option B

4. Quick Check:

   Use $in for multiple values in filter = C [OK]

Hint: Use $in operator to match multiple values in deleteMany filter [OK]

Common Mistakes:

• Using JavaScript OR inside filter object
• Misusing $or inside a field filter
• Passing array directly as field value