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.