Soft delete pattern in MongoDB - Mini Project: Build & Apply

Soft Delete Pattern in MongoDB

📖 Scenario: You are managing a user database for a web application. Instead of permanently deleting user records, you want to mark them as deleted so you can restore them later if needed.

🎯 Goal: Build a MongoDB collection with a soft delete pattern by adding a deleted field to user documents and write queries to filter out deleted users.

📋 What You'll Learn

Create a users collection with sample user documents

Add a deleted field to each user document to mark soft deletion

Write a query to find only users who are not deleted

Write a query to mark a user as deleted by setting deleted to true

💡 Why This Matters

🌍 Real World

Soft delete is used in many applications to avoid losing data permanently and to allow recovery of deleted records.

💼 Career

Understanding soft delete patterns is important for database management roles and backend development jobs where data integrity and recovery are critical.

1

Create the users collection with sample data

Create a users collection with these exact documents: { _id: 1, name: "Alice", deleted: false }, { _id: 2, name: "Bob", deleted: false }, and { _id: 3, name: "Charlie", deleted: false }.

MongoDB

// Insert the three user documents into the users collection
// Your code here

Hint

Use db.users.insertMany() to add multiple documents at once.

2

Add a helper variable for the soft delete filter

Create a variable called activeUsersFilter and set it to { deleted: false } to select only users who are not deleted.

MongoDB

db.users.insertMany([
  { _id: 1, name: "Alice", deleted: false },
  { _id: 2, name: "Bob", deleted: false },
  { _id: 3, name: "Charlie", deleted: false }
])

// Create a filter variable to find users where deleted is false
// Your code here

Hint

Use const activeUsersFilter = { deleted: false } to create the filter.

3

Write a query to find all active (not deleted) users

Write a query using db.users.find() with the filter variable activeUsersFilter to find all users where deleted is false.

MongoDB

db.users.insertMany([
  { _id: 1, name: "Alice", deleted: false },
  { _id: 2, name: "Bob", deleted: false },
  { _id: 3, name: "Charlie", deleted: false }
])

const activeUsersFilter = { deleted: false }

// Write a query to find all users where deleted is false
// Your code here

Hint

Use db.users.find(activeUsersFilter) to get active users.

4

Mark a user as deleted using an update query

Write an update query using db.users.updateOne() to set deleted to true for the user with _id equal to 2.

MongoDB

db.users.insertMany([
  { _id: 1, name: "Alice", deleted: false },
  { _id: 2, name: "Bob", deleted: false },
  { _id: 3, name: "Charlie", deleted: false }
])

const activeUsersFilter = { deleted: false }

const activeUsers = db.users.find(activeUsersFilter)

// Update the user with _id 2 to set deleted to true
// Your code here

Hint

Use db.users.updateOne({ _id: 2 }, { $set: { deleted: true } }) to mark the user as deleted.

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