Soft delete pattern in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the soft delete pattern in MongoDB?

Soft delete means marking a document as deleted without removing it from the database. This is usually done by adding a field like isDeleted: true instead of deleting the document.

beginner

Why use soft delete instead of hard delete?

Soft delete helps keep data for recovery, auditing, or history. It avoids losing data permanently and allows easy undo of deletions.

beginner

How do you query only active (not deleted) documents in MongoDB with soft delete?

Add a filter like { isDeleted: { $ne: true } } to your query to exclude documents marked as deleted.

beginner

What field is commonly added to implement soft delete in MongoDB?

A boolean field like isDeleted or a date field like deletedAt is added to mark deletion status.

intermediate

How can you permanently remove soft deleted documents later?

Use a query like db.collection.deleteMany({ isDeleted: true }) to clean up documents marked as deleted.

What does soft delete do in MongoDB?

A Marks documents as deleted without removing them

B Immediately removes documents from the database

C Backs up documents before deleting

D Encrypts documents before deletion

Soft delete marks documents as deleted (e.g., with isDeleted: true) but keeps them in the database.

Which field is commonly used to mark a document as deleted in soft delete?

A username

B createdAt

C isDeleted

D password

The isDeleted field is commonly added to indicate if a document is soft deleted.

How do you exclude soft deleted documents in a MongoDB query?

A { isDeleted: false }

B { isDeleted: true }

C { deletedAt: null }

D { isDeleted: { $ne: true } }

Using { isDeleted: { $ne: true } } filters out documents marked as deleted.

What is a benefit of using soft delete?

A Allows data recovery after deletion

B Makes queries faster

C Reduces database size

D Encrypts data automatically

Soft delete keeps data so you can recover it if needed.

How can you permanently remove soft deleted documents?

A Insert new documents

B Use deleteMany with { isDeleted: true }

C Update documents to isDeleted: false

D Use findOneAndUpdate

deleteMany with { isDeleted: true } removes all soft deleted documents permanently.

Explain the soft delete pattern in MongoDB and why it is useful.

Think about how you keep old files instead of throwing them away immediately.

Describe how to query only active documents when using soft delete in MongoDB.

How do you find files that are not marked 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