Why delete operations need care in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delete all documents where the field "status" is "inactive".

MongoDB

db.collection.deleteMany({"status": [1] })

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

A"inactive"

B"active"

Ctrue

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string value.

Using the wrong field name.

Using deleteOne instead of deleteMany when multiple documents match.

The deleteMany method requires a filter object. To delete documents where the status is "inactive", you must specify {"status": "inactive"}.

2fill in blank

medium

Complete the code to delete a single document where the "username" is "guest".

MongoDB

db.users.[1]({"username": "guest"})

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

Aremove

BdeleteMany

Cdrop

DdeleteOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using deleteMany and deleting more documents than intended.

Using drop which deletes the entire collection.

The deleteOne method deletes only the first document that matches the filter. Here, it deletes one user with username "guest".

3fill in blank

hard

Fix the error in the delete operation to avoid deleting all documents unintentionally.

MongoDB

db.orders.deleteMany([1])

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

A{"status": "pending"}

B{}

Cnull

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty filter and deleting the entire collection.

Passing null or an empty array as filter which causes errors.

Using an empty filter {} deletes all documents. To avoid this, specify a filter like {"status": "pending"} to delete only matching documents.

4fill in blank

hard

Fill both blanks to delete documents where "age" is greater than 30 and "active" is false.

MongoDB

db.users.deleteMany({"age": { [1]: 30 }, "active": [2] })

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

A"$gt"

Btrue

Cfalse

D"$lt"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$lt" instead of "$gt" for age comparison.

Using string "false" instead of boolean false.

To delete users older than 30, use "$gt" (greater than). To check if "active" is false, use the boolean false.

5fill in blank

hard

Fill all three blanks to delete documents where "score" is less than 50, "passed" is false, and "grade" is "F".

MongoDB

db.results.deleteMany({"score": { [1]: 50 }, "passed": [2], "grade": [3] })

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

A"$lt"

Bfalse

C"F"

D"$gt"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "$gt" instead of "$lt" for score.

Quoting boolean false as a string.

Forgetting quotes around grade "F".

Use "$lt" to find scores less than 50, boolean false for "passed", and string "F" for grade.

Practice

1. Why should you be careful when performing a delete operation in MongoDB?

easy

A. Because delete operations are slow and take a long time to complete.

B. Because delete operations only work on empty collections.

C. Because delete operations create duplicate data automatically.

D. Because deleted data is permanently removed and cannot be recovered easily.

Solution

1. Step 1: Understand the effect of delete operations

   Delete operations permanently remove documents from the database, meaning the data is lost unless backed up.

2. Step 2: Recognize the risk of permanent data loss

   Because data cannot be easily recovered after deletion, care is needed to avoid accidental loss.

3. Final Answer:

   Because deleted data is permanently removed and cannot be recovered easily. -> Option D

4. Quick Check:

   Delete = Permanent removal [OK]

Hint: Remember: delete means data is gone forever unless backed up [OK]

Common Mistakes:

• Thinking delete is slow by default
• Believing delete creates duplicates
• Assuming delete only works on empty collections

2. Which of the following is the correct syntax to delete a single document in MongoDB?

easy

A. db.collection.deleteOne({"name": "John"})

B. db.collection.removeOne({"name": "John"})

C. db.collection.delete({"name": "John"})

D. db.collection.deleteManyOne({"name": "John"})

Solution

1. Step 1: Recall MongoDB delete syntax

   The correct method to delete a single document is deleteOne() with a filter object.

2. Step 2: Check each option

   Only deleteOne() is a valid MongoDB method; others are invalid or do not exist.

3. Final Answer:

   db.collection.deleteOne({"name": "John"}) -> Option A

4. Quick Check:

   deleteOne() = single delete [OK]

Hint: Use deleteOne() to remove a single matching document [OK]

Common Mistakes:

• Using removeOne() which is not a valid method
• Using delete() which deletes all matching documents
• Confusing deleteManyOne() which does not exist

3. Given the following MongoDB commands, what will be the result count of documents after execution?

db.users.insertMany([{"name": "Alice"}, {"name": "Bob"}, {"name": "Alice"}])
db.users.deleteMany({"name": "Alice"})
db.users.find().count()

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Insert documents into the collection

   Three documents are inserted: two with name "Alice" and one with "Bob".

2. Step 2: Delete documents where name is "Alice"

   Both documents with "Alice" are deleted, leaving only the one with "Bob".

3. Step 3: Count remaining documents

   Only one document remains, so the count is 1.

4. Final Answer:

   1 -> Option C

5. Quick Check:

   3 inserted - 2 deleted = 1 left [OK]

Hint: Count after deleteMany equals original minus deleted matches [OK]

Common Mistakes:

• Assuming deleteMany deletes only one document
• Counting all inserted documents without deletion
• Confusing deleteMany with deleteOne

4. What is wrong with this MongoDB delete command?

db.products.deleteOne("category": "electronics")

medium

A. The filter is missing curly braces {}.

B. deleteOne cannot be used with a filter.

C. The collection name is incorrect.

D. The command should be deleteMany instead of deleteOne.

Solution

1. Step 1: Check the syntax of deleteOne

   The filter argument must be an object enclosed in curly braces {}.

2. Step 2: Identify the error in the command

   The filter is written without braces, causing a syntax error.

3. Final Answer:

   The filter is missing curly braces {}. -> Option A

4. Quick Check:

   Filter needs {} in deleteOne [OK]

Hint: Always wrap filter in {} for deleteOne [OK]

Common Mistakes:

• Omitting curly braces around filter
• Thinking deleteOne cannot take filters
• Confusing collection name with command

5. You want to delete all documents where the field status is either "inactive" or missing. Which MongoDB delete command correctly does this while avoiding accidental deletion of other documents?

hard

A. db.users.deleteMany({"status": {$in: ["inactive", null]}})

B. db.users.deleteMany({$or: [{"status": "inactive"}, {"status": {$exists: false}}]})

C. db.users.deleteMany({"status": "inactive", "status": {$exists: false}})

D. db.users.deleteMany({"status": "inactive" || {$exists: false}})

Solution

1. Step 1: Understand the filter requirements

   We want to delete documents where status is "inactive" OR status field does not exist.

2. Step 2: Analyze each option's filter

   db.users.deleteMany({$or: [{"status": "inactive"}, {"status": {$exists: false}}]}) uses $or with correct conditions; B and C have syntax errors; A checks for null but not missing field.

3. Step 3: Confirm correct syntax and logic

   db.users.deleteMany({$or: [{"status": "inactive"}, {"status": {$exists: false}}]}) correctly combines conditions with $or and uses $exists to check missing fields.

4. Final Answer:

   db.users.deleteMany({$or: [{"status": "inactive"}, {"status": {$exists: false}}]}) -> Option B

5. Quick Check:

   Use $or with $exists for missing fields [OK]

Hint: Use $or and $exists to target missing or specific values [OK]

Common Mistakes:

• Using || inside filter object (invalid syntax)
• Trying to combine conditions with commas incorrectly
• Using $in with null instead of $exists for missing fields