Soft delete pattern in MongoDB

Introduction

Soft delete lets you mark data as deleted without removing it. This helps keep data safe and recoverable.

When you want to keep a record of deleted items for audit or history.

When users might want to undo a delete action.

When you want to avoid accidental data loss.

When you need to keep related data intact without breaking references.

When you want to filter out deleted items but keep them in the database.

Syntax

MongoDB

db.collection.updateOne(
  { _id: ObjectId("id_here") },
  { $set: { deleted: true, deletedAt: new Date() } }
)

Use a boolean field like deleted to mark if an item is deleted.

Optionally add a deletedAt timestamp to know when it was deleted.

Examples

Marks a user as deleted by setting deleted to true and recording the time.

MongoDB

db.users.updateOne(
  { _id: ObjectId("507f1f77bcf86cd799439011") },
  { $set: { deleted: true, deletedAt: new Date() } }
)

Soft deletes all products in the "old" category.

MongoDB

db.products.updateMany(
  { category: "old" },
  { $set: { deleted: true, deletedAt: new Date() } }
)

Finds all orders that are not marked as deleted.

MongoDB

db.orders.find({ deleted: { $ne: true } })

Sample Program

This example soft deletes one product by setting deleted to true and adds a timestamp. Then it finds all products that are still active (not deleted).

MongoDB

use shopDB

// Soft delete a product by id
const productId = ObjectId("642f1f77bcf86cd799439012")
db.products.updateOne(
  { _id: productId },
  { $set: { deleted: true, deletedAt: new Date() } }
)

// Query products that are not deleted
db.products.find({ deleted: { $ne: true } })

Important Notes

Remember to always filter out documents where deleted is true when querying active data.

Soft delete keeps data safe but can increase database size over time.

You can add a background job to permanently delete old soft deleted data if needed.

Summary

Soft delete marks data as deleted without removing it.

Use a deleted boolean and optional deletedAt timestamp.

Always filter queries to exclude soft deleted data unless you want to see it.

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