updateMany method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this updateMany operation?

Consider a collection products with documents:

{ "_id": 1, "name": "Pen", "stock": 10 }

{ "_id": 2, "name": "Pencil", "stock": 5 }

{ "_id": 3, "name": "Eraser", "stock": 0 }

What will be the result of this command?

db.products.updateMany({ stock: { $lt: 10 } }, { $inc: { stock: 5 } })

MongoDB

db.products.updateMany({ stock: { $lt: 10 } }, { $inc: { stock: 5 } })

A {"acknowledged": true, "matchedCount": 1, "modifiedCount": 1}

B {"acknowledged": true, "matchedCount": 3, "modifiedCount": 3}

C {"acknowledged": true, "matchedCount": 2, "modifiedCount": 2}

D {"acknowledged": false, "matchedCount": 0, "modifiedCount": 0}

💡 Hint

Look for documents where stock is less than 10.

The filter { stock: { $lt: 10 } } matches documents with stock less than 10, which are the Pencil and Eraser. Both get their stock increased by 5, so matchedCount and modifiedCount are 2.

📝 Syntax

intermediate

Which updateMany command is syntactically correct?

Choose the correct syntax to update all documents where status is "pending" by setting status to "completed".

A db.orders.updateMany({ status == "pending" }, { $set: { status: "completed" } })

B db.orders.updateMany({ status: "pending" }, { $set: { status: "completed" } })

C db.orders.updateMany({ status: "pending" }, { status: "completed" })

D db.orders.updateMany({ status: "pending" }, { $update: { status: "completed" } })

💡 Hint

Remember the update document must use update operators like $set.

Option B uses the correct filter and update syntax with $set. Option B uses invalid filter syntax (==), Option B misses $set, and D uses a wrong update operator.

❓ optimization

advanced

How to optimize updateMany for large collections?

You want to update many documents in a large collection efficiently. Which approach improves performance?

A Add an index on the fields used in the filter condition before running updateMany.

B Run updateMany without any indexes to avoid overhead.

C Use updateMany with an empty filter to update all documents at once.

D Run multiple updateOne commands instead of updateMany.

💡 Hint

Indexes help queries find documents faster.

Adding an index on the filter fields helps MongoDB quickly find matching documents, making updateMany faster. Running without indexes or updating all documents blindly is slower. Multiple updateOne calls add overhead.

🔧 Debug

advanced

Why does this updateMany not modify any documents?

Given this command:

db.users.updateMany({ age: { $gt: 30 } }, { $set: { active: true } })

But no documents are modified even though some users have age 35 and 40. What is the likely cause?

MongoDB

db.users.updateMany({ age: { $gt: 30 } }, { $set: { active: true } })

A The collection <code>users</code> does not exist.

B The <code>$set</code> operator is missing in the update document.

C The filter syntax is invalid and causes an error.

D The <code>age</code> field is stored as a string, so the numeric filter fails.

💡 Hint

Check the data type of the age field in documents.

If age is stored as a string, the numeric comparison $gt: 30 does not match any documents, so no updates occur.

🧠 Conceptual

expert

What does updateMany return after execution?

After running updateMany, what information does the returned object contain?

A An object with <code>acknowledged</code>, <code>matchedCount</code>, and <code>modifiedCount</code> fields.

B The number of documents in the collection.

C A boolean indicating if the update was successful.

D An array of all updated documents.

💡 Hint

Think about what updateMany reports after running.

updateMany returns an object showing if the operation was acknowledged, how many documents matched the filter, and how many were actually modified.