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.

Practice

1. What is the main purpose of the soft delete pattern in MongoDB?

easy

A. To encrypt data before deletion

B. To permanently remove data immediately

C. To backup data before deletion

D. To mark data as deleted without actually removing it from the database

Solution

1. Step 1: Understand soft delete concept

   Soft delete means marking data as deleted but keeping it in the database.

2. Step 2: Compare options

   Only To mark data as deleted without actually removing it from the database describes marking data as deleted without removal.

3. Final Answer:

   To mark data as deleted without actually removing it from the database -> Option D

4. Quick Check:

   Soft delete = mark, not remove [OK]

Hint: Soft delete means mark deleted, not remove [OK]

Common Mistakes:

• Confusing soft delete with hard delete
• Thinking soft delete removes data
• Assuming soft delete encrypts data

2. Which of the following is the correct way to add a soft delete flag to a MongoDB document?

easy

A. { deleted: true }

B. { isDeleted: 'yes' }

C. { deletedAt: 'no' }

D. { remove: false }

Solution

1. Step 1: Identify common soft delete fields

   Soft delete usually uses a boolean field like 'deleted' set to true or false.

2. Step 2: Check options for correct boolean usage

   { deleted: true } uses { deleted: true } which is standard and correct.

3. Final Answer:

   { deleted: true } -> Option A

4. Quick Check:

   Soft delete flag = boolean true [OK]

Hint: Use boolean field deleted: true for soft delete [OK]

Common Mistakes:

• Using string values instead of boolean
• Using unrelated field names
• Confusing deletedAt with boolean flag

3. Given the collection documents:
{ _id: 1, name: 'Alice', deleted: false }
{ _id: 2, name: 'Bob', deleted: true }
What will this query return?
db.users.find({ deleted: false })

medium

A. [{ _id: 1, name: 'Alice', deleted: false }]

B. [{ _id: 2, name: 'Bob', deleted: true }]

C. []

D. All documents

Solution

1. Step 1: Understand the query filter

   The query filters documents where deleted is false.

2. Step 2: Check documents matching filter

   Only the document with _id 1 has deleted: false, so it is returned.

3. Final Answer:

   [{ _id: 1, name: 'Alice', deleted: false }] -> Option A

4. Quick Check:

   Filter deleted: false returns Alice [OK]

Hint: Filter deleted: false to exclude soft deleted [OK]

Common Mistakes:

• Returning documents with deleted: true
• Returning empty result incorrectly
• Assuming query returns all documents

4. You want to update a document to soft delete it by setting deleted: true. Which of these update commands is correct?

medium

A. db.collection.updateOne({ _id: 1 }, { deleted: true })

B. db.collection.updateOne({ _id: 1 }, { $set: { deleted: true } })

C. db.collection.updateOne({ _id: 1 }, { $unset: { deleted: true } })

D. db.collection.updateOne({ _id: 1 }, { $push: { deleted: true } })

Solution

1. Step 1: Recall MongoDB update syntax

   To update a field, use $set operator with the new value.

2. Step 2: Analyze options

   db.collection.updateOne({ _id: 1 }, { $set: { deleted: true } }) correctly uses $set to set deleted: true. db.collection.updateOne({ _id: 1 }, { deleted: true }) misses $set, causing replacement. Options C and D use wrong operators.

3. Final Answer:

   db.collection.updateOne({ _id: 1 }, { $set: { deleted: true } }) -> Option B

4. Quick Check:

   Use $set to update fields [OK]

Hint: Use $set to update fields in MongoDB [OK]

Common Mistakes:

• Omitting $set causing document replacement
• Using $unset instead of $set
• Using $push on non-array field

5. You want to find all documents including soft deleted ones, but sort them so that non-deleted come first. Which query achieves this?

hard

A. db.collection.find({ deleted: { $exists: false } })

B. db.collection.find({ deleted: false }).sort({ name: 1 })

C. db.collection.find().sort({ deleted: 1 })

D. db.collection.find().sort({ deleted: -1 })

Solution

1. Step 1: Understand requirement

   We want all documents, including deleted, but sorted so deleted: false first.

2. Step 2: Analyze sorting by deleted field

   Sorting by deleted: 1 sorts false (0) before true (1), so non-deleted come first.

3. Step 3: Check options

   db.collection.find().sort({ deleted: 1 }) finds all and sorts by deleted ascending, matching requirement. db.collection.find({ deleted: false }).sort({ name: 1 }) filters out deleted documents. db.collection.find().sort({ deleted: -1 }) sorts deleted descending (deleted first). db.collection.find({ deleted: { $exists: false } }) filters documents missing deleted field.

4. Final Answer:

   db.collection.find().sort({ deleted: 1 }) -> Option C

5. Quick Check:

   Sort by deleted ascending puts non-deleted first [OK]

Hint: Sort by deleted: 1 to put non-deleted first [OK]

Common Mistakes:

• Filtering out deleted documents instead of including all
• Sorting deleted descending to put deleted first
• Filtering by missing deleted field