Soft delete pattern in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mark a document as deleted by setting the 'deleted' field to true.

MongoDB

db.users.updateOne({ _id: userId }, { $set: { deleted: [1] } })

Drag options to blanks, or click blank then click option'

Aundefined

Bfalse

Cnull

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 'deleted' to false instead of true.

Using null or undefined which do not indicate deletion.

Setting the 'deleted' field to true marks the document as soft deleted.

2fill in blank

medium

Complete the query to find only documents that are not soft deleted.

MongoDB

db.users.find({ deleted: [1] })

Drag options to blanks, or click blank then click option'

Atrue

Bnull

Cfalse

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Searching for deleted: true which returns deleted documents.

Using null or empty object which does not filter correctly.

Documents with 'deleted' set to false are active (not deleted).

3fill in blank

hard

Fix the error in the update query to soft delete a document by setting 'deleted' to true.

MongoDB

db.users.updateOne({ _id: userId }, { $set: { deleted: [1] } })

Drag options to blanks, or click blank then click option'

Anull

Btrue

C1

D"true"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "true" as a string instead of boolean true.

Using 1 which is a number, not a boolean.

The value should be the boolean true, not a string or number.

4fill in blank

hard

Fill both blanks to query active users and sort them by creation date descending.

MongoDB

db.users.find({ deleted: [1] }).sort({ createdAt: [2] })

Drag options to blanks, or click blank then click option'

Afalse

Btrue

C-1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using deleted: true to find deleted users.

Sorting ascending (1) instead of descending (-1).

We find users where 'deleted' is false (active users) and sort by 'createdAt' descending using -1.

5fill in blank

hard

Fill all three blanks to update a document's 'deleted' status, set 'deletedAt' timestamp, and exclude deleted documents in a query.

MongoDB

db.users.updateOne({ _id: userId }, { $set: { deleted: [1], deletedAt: [2] } });
db.users.find({ deleted: [3] })

Drag options to blanks, or click blank then click option'

Atrue

Bnew Date()

Cfalse

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Not setting deletedAt timestamp.

Querying for deleted: true instead of false.

Set 'deleted' to true and 'deletedAt' to the current date. Then query documents where 'deleted' is false to exclude deleted ones.

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