deleteOne method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this deleteOne operation?

Consider a MongoDB collection named users with documents:

{ "_id": 1, "name": "Alice" }

{ "_id": 2, "name": "Bob" }

What will be the result of this command?

db.users.deleteOne({ name: "Alice" })

MongoDB

db.users.deleteOne({ name: "Alice" })

A {"acknowledged": true, "deletedCount": 1}

B {"acknowledged": true, "deletedCount": 0}

C SyntaxError: missing parentheses

D TypeError: deleteOne is not a function

💡 Hint

deleteOne removes the first document matching the filter.

The deleteOne method deletes one document matching the filter. Since a document with name 'Alice' exists, deletedCount is 1.

🧠 Conceptual

intermediate

Which statement about deleteOne is true?

Choose the correct statement about the MongoDB deleteOne method.

A deleteOne deletes exactly one document matching the filter.

B deleteOne deletes all documents matching the filter.

C deleteOne returns the deleted document.

D deleteOne requires the document _id to delete.

💡 Hint

Think about how many documents deleteOne removes.

deleteOne deletes only the first document that matches the filter. It does not delete all matching documents or return the deleted document.

📝 Syntax

advanced

Which deleteOne command is syntactically correct?

Identify the correct syntax for deleting one document where age is 30.

A db.collection.deleteOne[ { age: 30 } ]

B db.collection.deleteOne({ age: 30 })

C db.collection.deleteOne({age:30}

D db.collection.deleteOne(age == 30)

💡 Hint

deleteOne takes a filter object inside parentheses.

Option B uses correct syntax: method call with a filter object inside parentheses. Others have syntax errors.

🔧 Debug

advanced

Why does this deleteOne command fail?

Given the command:

db.users.deleteOne({ "name": "Charlie" })

and the error:

TypeError: db.users.deleteOne is not a function

What is the most likely cause?

A The filter object is missing required fields.

B The deleteOne method is misspelled.

C The database connection is not established properly.

D The collection name is incorrect or does not exist.

💡 Hint

Verify the database connection and collection reference.

The TypeError indicates that db.users does not have deleteOne as a function, which typically occurs when the database connection is not established properly or the shell/context is incorrect.

❓ optimization

expert

How to optimize deleting a document by _id using deleteOne?

You want to delete a document by its unique _id field efficiently. Which option is the best practice?

A db.collection.deleteOne({ _id: new ObjectId("507f1f77bcf86cd799439011") })

B db.collection.deleteOne({ _id: "507f1f77bcf86cd799439011" })

C db.collection.deleteOne({ id: ObjectId("507f1f77bcf86cd799439011") })

D db.collection.deleteOne({ _id: ObjectId("507f1f77bcf86cd799439011") })

💡 Hint

The _id field is an ObjectId type, not a string.

The _id field must be matched with an ObjectId instance. Option D uses ObjectId correctly. Option D uses a string, which won't match. Option D uses wrong field name. Option D uses new ObjectId which is not standard in shell.

Practice

1. What does the deleteOne method do in MongoDB?

easy

A. Deletes all documents in a collection.

B. Updates a single document in the collection.

C. Deletes a single document that matches the filter criteria.

D. Finds a document without deleting it.

Solution

1. Step 1: Understand the purpose of deleteOne

   The deleteOne method is designed to remove exactly one document that matches the given filter.

2. Step 2: Compare with other methods

   Unlike deleteMany, which removes multiple documents, deleteOne targets only one document.

3. Final Answer:

   Deletes a single document that matches the filter criteria. -> Option C

4. Quick Check:

   deleteOne removes one matching document [OK]

Hint: Remember: deleteOne removes only one matching document [OK]

Common Mistakes:

• Confusing deleteOne with deleteMany
• Thinking deleteOne updates documents
• Assuming deleteOne deletes all documents

2. Which of the following is the correct syntax to delete one document where the field name equals "Alice"?

easy

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

B. db.collection.deleteOne(name = "Alice")

C. db.collection.deleteOne("name: Alice")

D. db.collection.deleteOne({"name" == "Alice"})

Solution

1. Step 1: Check the filter format

   The filter must be an object with key-value pairs, like {name: "Alice"}.

2. Step 2: Validate method call syntax

   The correct syntax is db.collection.deleteOne(filter) where filter is an object.

3. Final Answer:

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

4. Quick Check:

   Filter is an object with field and value [OK]

Hint: Use object syntax {field: value} inside deleteOne() [OK]

Common Mistakes:

• Using assignment (=) inside filter
• Passing filter as a string
• Using comparison operators (==) inside filter object

3. Given the collection documents: [{"_id":1, "name":"Bob"}, {"_id":2, "name":"Alice"}, {"_id":3, "name":"Bob"}], what will be the result of db.collection.deleteOne({name: "Bob"})?

medium

A. Deletes the first document with name 'Bob' only.

B. No documents are deleted.

C. Deletes the document with name 'Alice'.

D. Deletes both documents with name 'Bob'.

Solution

1. Step 1: Understand deleteOne behavior

   deleteOne removes only one document matching the filter, not all.

2. Step 2: Identify which document is deleted

   It deletes the first document it finds with name: "Bob", which is the one with _id:1.

3. Final Answer:

   Deletes the first document with name 'Bob' only. -> Option A

4. Quick Check:

   deleteOne removes one matching document, not all [OK]

Hint: deleteOne deletes only one matching document, not all [OK]

Common Mistakes:

• Assuming deleteOne deletes all matches
• Thinking it deletes documents with other names
• Believing no document is deleted if multiple matches exist

4. What is wrong with this code snippet?
db.collection.deleteOne({name: 'John')

medium

A. deleteOne cannot delete documents with name field.

B. Using single quotes instead of double quotes.

C. The method name should be deleteMany.

D. Missing closing brace '}' in the filter object.

Solution

1. Step 1: Check the filter object syntax

   The filter object starts with '{' but does not have a matching closing '}'.

2. Step 2: Validate quotes and method usage

   Single quotes are allowed in JavaScript, and deleteOne is correct for deleting one document.

3. Final Answer:

   Missing closing brace '}' in the filter object. -> Option D

4. Quick Check:

   Filter objects must have matching braces [OK]

Hint: Check for matching braces in filter objects [OK]

Common Mistakes:

• Ignoring missing braces causing syntax errors
• Confusing single vs double quotes as error
• Thinking deleteOne is wrong method here

5. You want to delete a user document only if the user's status is "inactive" and age is greater than 30. Which deleteOne filter correctly achieves this?

hard

A. db.collection.deleteOne({$or: [{status: "inactive"}, {age: {$gt: 30}}]})

B. db.collection.deleteOne({status: "inactive", age: {$gt: 30}})

C. db.collection.deleteOne({status == "inactive", age > 30})

D. db.collection.deleteOne({status: "inactive" || age > 30})

Solution

1. Step 1: Understand filter conditions

   We want to delete a document where both conditions are true: status is "inactive" AND age is greater than 30.

2. Step 2: Check filter syntax for AND condition

   In MongoDB filters, multiple key-value pairs in the object imply logical AND. Thus, db.collection.deleteOne({status: "inactive", age: {$gt: 30}}) is correct. $or would match if either condition is true.

3. Final Answer:

   db.collection.deleteOne({status: "inactive", age: {$gt: 30}}) -> Option B

4. Quick Check:

   Multiple fields in filter imply AND [OK]

Hint: Multiple fields in filter object mean AND conditions [OK]

Common Mistakes:

• Using || inside filter object (invalid syntax)
• Using == inside filter object
• Using $or for AND conditions