Soft delete pattern in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find active users excluding soft deleted ones

Given a MongoDB collection users where soft deleted documents have deletedAt field set to a timestamp, which query returns only active (not soft deleted) users?

MongoDB

db.users.find({ deletedAt: { $exists: false } })

A db.users.find({ deletedAt: { $exists: true } })

B db.users.find({ deletedAt: null })

C db.users.find({ deletedAt: { $ne: null } })

D db.users.find({ deletedAt: { $exists: false } })

💡 Hint

Think about how to check if a field is missing in MongoDB documents.

Soft deleted documents have the deletedAt field set. Active documents do not have this field. Using $exists: false filters documents where deletedAt is missing.

❓ query_result

intermediate

Count soft deleted documents

How to count the number of soft deleted documents in a collection products where soft delete is indicated by a deletedAt field with a timestamp?

MongoDB

db.products.countDocuments({ deletedAt: { $exists: true } })

A db.products.countDocuments({ deletedAt: { $exists: true } })

B db.products.countDocuments({ deletedAt: { $eq: null } })

C db.products.countDocuments({ deletedAt: { $exists: false } })

D db.products.countDocuments({ deletedAt: null })

💡 Hint

Soft deleted documents have the deletedAt field present.

Counting documents where deletedAt exists counts all soft deleted documents.

📝 Syntax

advanced

Update document to soft delete

Which update query correctly marks a document as soft deleted by setting deletedAt to the current date?

MongoDB

db.orders.updateOne({ _id: 123 }, { $set: { deletedAt: new Date() } })

A db.orders.updateOne({ _id: 123 }, { $set: { deletedAt: new Date() } })

B db.orders.updateOne({ _id: 123 }, { $set: deletedAt: new Date() })

C db.orders.updateOne({ _id: 123 }, { deletedAt: new Date() })

D db.orders.updateOne({ _id: 123 }, { $push: { deletedAt: new Date() } })

💡 Hint

Remember the correct syntax for MongoDB update operators.

The $set operator is used to assign a new value to a field. Option A uses the correct syntax.

🧠 Conceptual

advanced

Why use soft delete instead of hard delete?

What is the main advantage of using soft delete (marking documents as deleted) over hard delete (removing documents) in MongoDB?

A Soft delete automatically archives documents to another collection.

B Soft delete allows recovery of deleted data if needed later.

C Soft delete improves query performance by reducing document count.

D Soft delete encrypts deleted documents for security.

💡 Hint

Think about data safety and recovery.

Soft delete keeps the data in the database, allowing recovery or auditing later. Hard delete removes data permanently.

❓ optimization

expert

Optimize queries with soft delete filter

You have a large customers collection with many soft deleted documents marked by deletedAt. Which index improves performance for queries filtering active customers (deletedAt missing)?

A { deletedAt: -1 }

B { deletedAt: 1, status: 1 }

C { deletedAt: 1 }

D { deletedAt: 1, _id: 1 }

💡 Hint

Think about indexing the field used in the filter.

Indexing deletedAt helps MongoDB quickly find documents where deletedAt does not exist or is null, improving query speed.