updateOne method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this updateOne query do?

Consider a MongoDB collection named users with documents containing name and age. What will be the result of this query?

db.users.updateOne({ name: "Alice" }, { $set: { age: 30 } })

MongoDB

db.users.updateOne({ name: "Alice" }, { $set: { age: 30 } })

A Updates all documents where name is 'Alice' by setting age to 30

B Updates the first document where name is 'Alice' by setting age to 30

C Inserts a new document with name 'Alice' and age 30 if none exists

D Deletes the first document where name is 'Alice'

💡 Hint

Remember, updateOne affects only one matching document.

The updateOne method updates only the first document matching the filter. It sets the age field to 30 for the first user named 'Alice'. It does not update multiple documents or insert new ones.

📝 Syntax

intermediate

Which updateOne query is syntactically correct?

Choose the correct syntax for updating the status field to 'active' for the user with userId 123.

A db.users.updateOne({ userId: 123 }, { status: 'active' })

B db.users.updateOne({ userId: 123 }, { set: { status: 'active' } })

C db.users.updateOne({ userId: 123 }, { $update: { status: 'active' } })

D db.users.updateOne({ userId: 123 }, { $set: { status: 'active' } })

💡 Hint

Use the correct update operator to modify fields.

The $set operator is required to update specific fields in a document. Options A, B, and D either miss the operator or use incorrect ones.

❓ optimization

advanced

Optimizing updateOne with upsert option

You want to update the score of a player with playerId 42. If no such player exists, insert a new document with playerId 42 and score 100. Which query achieves this efficiently?

A db.players.updateOne({ playerId: 42 }, { $set: { score: 100 } }, { upsert: true })

B db.players.updateMany({ playerId: 42 }, { $set: { score: 100 } }, { upsert: true })

C db.players.insertOne({ playerId: 42, score: 100 })

D db.players.updateOne({ playerId: 42 }, { $set: { score: 100 } })

💡 Hint

Use the option that inserts if no match is found.

The upsert option in updateOne inserts a new document if no matching document exists. updateMany is not needed here, and insertOne does not update existing documents.

🔧 Debug

advanced

Why does this updateOne query fail?

Given the query:

db.orders.updateOne({ orderId: 101 }, { status: 'shipped' })

It does not update the document as expected. What is the cause?

A Missing $set operator in the update document

B orderId field does not exist in the collection

C updateOne cannot update string fields

D The filter object is empty

💡 Hint

Check the update document structure.

The update document must use an update operator like $set. Without it, MongoDB treats the document as a replacement, which is invalid here.

🧠 Conceptual

expert

What is the difference between updateOne and findOneAndUpdate?

Which statement correctly describes a key difference between updateOne and findOneAndUpdate methods in MongoDB?

A updateOne returns the updated document, findOneAndUpdate returns only the update result

B Both methods behave identically in all cases

C findOneAndUpdate returns the original or updated document, updateOne returns only update status

D updateOne updates multiple documents, findOneAndUpdate updates only one

💡 Hint

Consider what each method returns after updating.

updateOne returns a result object with status info but not the document. findOneAndUpdate returns the document before or after update depending on options.

Practice

1. What does the updateOne method do in MongoDB?

easy

A. Inserts a new document without checking existing ones.

B. Deletes all documents matching the filter.

C. Returns all documents without any update.

D. Updates a single document that matches the filter criteria.

Solution

1. Step 1: Understand the purpose of updateOne

   The updateOne method is designed to update only one document that matches the given filter.

2. Step 2: Compare with other operations

   Deleting or inserting documents are different operations; updateOne specifically updates one matching document.

3. Final Answer:

   Updates a single document that matches the filter criteria. -> Option D

4. Quick Check:

   updateOne updates one document [OK]

Hint: Remember: updateOne changes only one matching document [OK]

Common Mistakes:

• Thinking updateOne deletes documents
• Confusing updateOne with insert operations
• Assuming updateOne updates multiple documents

2. Which of the following is the correct syntax to update the field age to 30 using updateOne?

easy

A. db.collection.updateOne({name: 'John'}, {$change: {age: 30}});

B. db.collection.updateOne({name: 'John'}, {$set: {age: 30}});

C. db.collection.updateOne({name: 'John'}, {$update: {age: 30}});

D. db.collection.updateOne({name: 'John'}, {age: 30});

Solution

1. Step 1: Identify the update operator

   MongoDB requires using $set to update fields safely without replacing the whole document.

2. Step 2: Check syntax correctness

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}); uses $set correctly; other options use invalid operators or omit $set.

3. Final Answer:

   db.collection.updateOne({name: 'John'}, {$set: {age: 30}}); -> Option B

4. Quick Check:

   Use $set to update fields [OK]

Hint: Always use $set inside updateOne to change fields [OK]

Common Mistakes:

• Omitting $set and passing fields directly
• Using wrong update operators like $update or $change
• Forgetting to wrap update fields in an object

3. Given the collection documents:
{ _id: 1, name: 'Alice', age: 25 }
{ _id: 2, name: 'Bob', age: 28 }
What will be the result of this operation?
db.collection.updateOne({name: 'Alice'}, {$set: {age: 26}});
Then querying db.collection.find({name: 'Alice'})?

medium

A. [{ _id: 1, name: 'Alice', age: 26 }]

B. [{ _id: 1, name: 'Alice', age: 25 }]

C. No documents found

D. [{ _id: 2, name: 'Bob', age: 28 }]

Solution

1. Step 1: Understand updateOne effect

   The updateOne matches the document with name: 'Alice' and updates the age to 26.

2. Step 2: Query after update

   Querying for name: 'Alice' returns the updated document with age: 26.

3. Final Answer:

   [{ _id: 1, name: 'Alice', age: 26 }] -> Option A

4. Quick Check:

   updateOne changes age to 26 [OK]

Hint: updateOne changes one matching document's fields [OK]

Common Mistakes:

• Expecting no change after updateOne
• Confusing which document is updated
• Assuming updateOne updates multiple documents

4. What is wrong with this updateOne command?
db.collection.updateOne({name: 'Eve'}, {age: 35});

medium

A. The collection name is invalid.

B. Filter syntax is incorrect.

C. Missing $set operator to update the field.

D. The updateOne method cannot update numeric fields.

Solution

1. Step 1: Check update document structure

   The update document must use an update operator like $set to modify fields.

2. Step 2: Identify missing operator

   The command directly passes {age: 35} without $set, which replaces the entire document instead of just updating the age field.

3. Final Answer:

   Missing $set operator to update the field. -> Option C

4. Quick Check:

   Always use $set in updateOne updates [OK]

Hint: Always include $set when updating fields [OK]

Common Mistakes:

• Forgetting $set operator
• Assuming updateOne accepts direct field objects
• Misunderstanding filter vs update parts

5. You want to update the document with username: 'mike' to set active: true. If no such document exists, you want to create it with username: 'mike' and active: true. Which updateOne command achieves this?

hard

A. db.collection.updateOne({username: 'mike'}, {$set: {active: true}}, {upsert: true});

B. db.collection.updateOne({username: 'mike'}, {$set: {active: true}});

C. db.collection.updateOne({username: 'mike'}, {active: true}, {upsert: true});

D. db.collection.updateOne({username: 'mike'}, {$set: {active: true}}, {insertIfNotFound: true});

Solution

1. Step 1: Use upsert option for insert if no match

   The upsert: true option tells MongoDB to insert if no document matches the filter.

2. Step 2: Use $set to update or create fields

   The update uses $set to set active: true. The filter ensures username: 'mike' is matched or inserted.

3. Final Answer:

   db.collection.updateOne({username: 'mike'}, {$set: {active: true}}, {upsert: true}); -> Option A

4. Quick Check:

   upsert true + $set updates or inserts [OK]

Hint: Use upsert: true with $set to update or insert [OK]

Common Mistakes:

• Omitting upsert option
• Passing update fields without $set
• Using wrong option name like insertIfNotFound