Upsert behavior (update or insert) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this upsert operation?

Consider a MongoDB collection users with documents:

{ _id: 1, name: "Alice", age: 25 }
{ _id: 2, name: "Bob", age: 30 }

What will be the state of the document with _id: 3 after running this update with upsert?

db.users.updateOne({ _id: 3 }, { $set: { name: "Charlie", age: 22 } }, { upsert: true })

MongoDB

db.users.updateOne({ _id: 3 }, { $set: { name: "Charlie", age: 22 } }, { upsert: true })

A The document with _id: 3 is deleted if it exists; otherwise, a new document is inserted.

B The document with _id: 3 is updated if it exists; otherwise, no change occurs.

C An error is thrown because _id: 3 does not exist.

D A new document { _id: 3, name: "Charlie", age: 22 } is inserted.

💡 Hint

Upsert means update if found, insert if not found.

When using upsert:true, if no document matches the filter, MongoDB inserts a new document combining the filter and update fields. Here, _id:3 does not exist, so a new document with _id:3 and the specified fields is inserted.

🧠 Conceptual

intermediate

Which statement best describes the upsert option in MongoDB?

Choose the most accurate description of what the upsert option does in MongoDB update operations.

A It updates the first document matching the filter or inserts a new document if none match.

B It only inserts a new document and never updates existing ones.

C It updates all documents matching the filter and inserts a new document if none match.

D It deletes documents matching the filter and inserts a new document.

💡 Hint

Upsert combines update and insert behavior.

The upsert option updates the first matching document if found; if no document matches, it inserts a new document based on the filter and update.

📝 Syntax

advanced

Which updateOne command correctly performs an upsert to set age to 40 for user with _id 5?

Select the MongoDB command that correctly updates the age to 40 for the document with _id 5, inserting it if it does not exist.

A db.users.updateOne({ _id: 5 }, { age: 40 }, { upsert: true })

B db.users.updateOne({ _id: 5 }, { $set: { age: 40 } }, { upsert: true })

C db.users.updateOne({ _id: 5 }, { $set: age: 40 }, { upsert: true })

D db.users.updateOne({ _id: 5 }, { $update: { age: 40 } }, { upsert: true })

💡 Hint

Update operators like $set are required to modify fields.

The update document must use an update operator like $set to modify fields. Option B correctly uses $set. Option B lacks $set and is invalid. Option B has syntax error in $set usage. Option B uses a non-existent operator $update.

🔧 Debug

advanced

Why does this upsert operation fail with a duplicate key error?

Given a collection with a unique index on the field email, what causes this error?

db.users.updateOne({ email: "[e-mail]" }, { $set: { name: "New Name" } }, { upsert: true })

Assume a document with email: "[e-mail]" already exists.

A The $set operator cannot be used with upsert.

B The update document is missing the _id field required for upsert.

C The upsert tries to insert a new document with duplicate email violating the unique index.

D The filter matches multiple documents causing a duplicate key error.

💡 Hint

Unique indexes prevent duplicate values on indexed fields.

If the filter does not match any document, upsert inserts a new document combining filter and update. If the filter matches multiple documents, updateOne updates the first. Here, the filter matches an existing document, so update occurs. But if the filter is incorrect or the update tries to insert a duplicate email, a duplicate key error occurs.

❓ optimization

expert

How to optimize bulk upsert operations in MongoDB for large datasets?

You need to perform thousands of upsert operations efficiently on a MongoDB collection. Which approach is best to optimize performance?

A Use bulkWrite with updateOne operations including upsert:true in a single batch.

B Insert all documents and catch duplicate key errors to update existing ones.

C Run individual updateOne operations with upsert:true in a loop.

D Delete all documents first, then insert all new documents.

💡 Hint

Batch operations reduce network overhead and improve speed.

Using bulkWrite with updateOne and upsert:true batches many operations into one request, reducing network calls and improving performance. Running individual updates is slower. Inserting and catching errors is inefficient. Deleting all data is risky and costly.

Practice

1. What does the upsert option do in a MongoDB updateOne operation?

easy

A. It only inserts a new document without updating existing ones.

B. It updates a document if it exists, or inserts a new one if it does not.

C. It deletes a document if it exists, otherwise does nothing.

D. It duplicates the document regardless of existence.

Solution

1. Step 1: Understand the upsert option

   The upsert option in MongoDB means update if found, insert if not found.

2. Step 2: Apply to updateOne operation

   When using updateOne with upsert: true, MongoDB updates the matching document or inserts a new one if none matches.

3. Final Answer:

   It updates a document if it exists, or inserts a new one if it does not. -> Option B

4. Quick Check:

   Upsert = update or insert [OK]

Hint: Upsert means update if found, else insert new [OK]

Common Mistakes:

• Thinking upsert only inserts without updating
• Confusing upsert with delete operation
• Assuming upsert duplicates documents

2. Which of the following is the correct syntax to perform an upsert using updateOne in MongoDB?

easy

A. db.collection.updateOne(filter, update, {update: true})

B. db.collection.updateOne(filter, update, {insert: true})

C. db.collection.updateOne(filter, update, {upsert: true})

D. db.collection.updateOne(filter, update, {replace: true})

Solution

1. Step 1: Recall the updateOne method parameters

   The updateOne method takes a filter, an update document, and an options object.

2. Step 2: Identify the correct option for upsert

   The option to enable upsert is {upsert: true}, which tells MongoDB to insert if no match is found.

3. Final Answer:

   db.collection.updateOne(filter, update, {upsert: true}) -> Option C

4. Quick Check:

   Use upsert: true in options [OK]

Hint: Use {upsert: true} in updateOne options [OK]

Common Mistakes:

• Using {insert: true} instead of {upsert: true}
• Omitting the options object entirely
• Confusing update with replace option

3. Given the collection users with documents:
{ _id: 1, name: "Alice", age: 25 }
What will be the result after running:

db.users.updateOne({ _id: 2 }, { $set: { name: "Bob", age: 30 } }, { upsert: true })

and then querying db.users.find().toArray()?

medium

A. [{ _id: 1, name: "Alice", age: 25 }, { _id: 2, name: "Bob", age: 30 }]

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

C. [{ _id: 2, name: "Bob", age: 30 }]

D. []

Solution

1. Step 1: Understand the updateOne with upsert

   The filter looks for _id: 2, which does not exist, so upsert inserts a new document with _id: 2 and the given fields.

2. Step 2: Check existing documents

   The original document with _id: 1 remains unchanged, so the collection now has two documents.

3. Final Answer:

   [{ _id: 1, name: "Alice", age: 25 }, { _id: 2, name: "Bob", age: 30 }] -> Option A

4. Quick Check:

   Upsert inserts missing document, keeps existing [OK]

Hint: Upsert inserts new if filter misses existing docs [OK]

Common Mistakes:

• Assuming existing documents get deleted
• Thinking upsert only updates existing documents
• Expecting only the new document after upsert

4. You run the following code but it does not insert a new document when no match is found:

db.products.updateOne({ sku: "123" }, { price: 19.99 })

What is the likely error?

medium

A. The filter syntax is incorrect and causes a syntax error.

B. MongoDB does not support upsert with updateOne.

C. The update document is missing the $set operator.

D. Missing the upsert: true option in the updateOne call.

Solution

1. Step 1: Check the updateOne parameters

   The updateOne call lacks the options object with upsert: true, so it only updates existing documents.

2. Step 2: Confirm upsert behavior

   Without upsert: true, no new document is inserted if the filter finds no match.

3. Final Answer:

   Missing the upsert: true option in the updateOne call. -> Option D

4. Quick Check:

   Upsert option needed to insert new docs [OK]

Hint: Add {upsert: true} to updateOne options to insert [OK]

Common Mistakes:

• Forgetting to add upsert option
• Confusing missing $set with upsert behavior
• Believing updateOne cannot upsert

5. You want to update the status field to "active" for a user with email: "[e-mail]". If no such user exists, insert a new document with email and status. Which MongoDB command correctly achieves this?

hard

A. db.users.updateOne({ email: "[e-mail]" }, { $set: { status: "active" } }, { upsert: true })

B. db.users.insertOne({ email: "[e-mail]", status: "active" })

C. db.users.updateOne({ email: "[e-mail]" }, { status: "active" }, { upsert: true })

D. db.users.updateMany({ email: "[e-mail]" }, { $set: { status: "active" } })

Solution

1. Step 1: Identify the correct update syntax with upsert

   To update or insert, use updateOne with a filter, an update using $set, and {upsert: true} option.

2. Step 2: Check each option

   db.users.updateOne({ email: "[e-mail]" }, { $set: { status: "active" } }, { upsert: true }) uses $set and upsert correctly. db.users.insertOne({ email: "[e-mail]", status: "active" }) only inserts, no update. db.users.updateOne({ email: "[e-mail]" }, { status: "active" }, { upsert: true }) misses $set operator, which replaces the whole document incorrectly. db.users.updateMany({ email: "[e-mail]" }, { $set: { status: "active" } }) updates many but no upsert.

3. Final Answer:

   db.users.updateOne({ email: "[e-mail]" }, { $set: { status: "active" } }, { upsert: true }) -> Option A

4. Quick Check:

   Use updateOne + $set + upsert: true [OK]

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

Common Mistakes:

• Omitting $set causing document replacement
• Using insertOne which does not update
• Forgetting upsert option for insert fallback