$nin for not in set in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where 'status' is NOT in a given set

Given a collection orders with documents containing a status field, which query returns all orders where the status is NOT 'shipped', 'delivered', or 'returned'?

MongoDB

db.orders.find({ status: { $nin: ['shipped', 'delivered', 'returned'] } })

A db.orders.find({ status: { $not: { $in: ['shipped', 'delivered', 'returned'] } } })

B db.orders.find({ status: { $in: ['shipped', 'delivered', 'returned'] } })

C db.orders.find({ status: { $nin: ['shipped', 'delivered', 'returned'] } })

D db.orders.find({ status: { $ne: ['shipped', 'delivered', 'returned'] } })

💡 Hint

Use $nin to exclude documents where the field matches any value in the array.

The $nin operator selects documents where the field's value is not in the specified array. Options B and D select documents where status is in or not equal to the entire array (which is invalid). Option C uses $not incorrectly.

🧠 Conceptual

intermediate

Understanding $nin behavior with missing fields

What happens when you use $nin on a field that does not exist in some documents?

A Documents missing the field are included only if $exists is true.

B Documents missing the field are excluded because $nin requires the field to exist.

C Documents missing the field cause a query error.

D Documents missing the field are included because their value is not in the $nin array.

💡 Hint

Think about how MongoDB treats missing fields in queries.

When a field is missing, MongoDB treats it as if the field's value is null or undefined. Since null is not in the $nin array, those documents match the query.

📝 Syntax

advanced

Identify the syntax error in $nin usage

Which of the following MongoDB queries has a syntax error when using $nin?

A db.collection.find({ age: { $nin: [25, 30, 35] } })

B db.collection.find({ age: { $nin: 25, 30, 35 } })

C db.collection.find({ age: { $nin: ['25', '30', '35'] } })

D db.collection.find({ age: { $nin: [] } })

💡 Hint

Check the syntax of the array passed to $nin.

Option B is invalid because $nin expects an array as its value, but here multiple values are passed without array brackets, causing a syntax error.

❓ optimization

advanced

Optimizing a query using $nin with indexes

You have a large collection with an index on the category field. Which query is more efficient to find documents where category is NOT 'A', 'B', or 'C'?

A db.products.find({ category: { $nin: ['A', 'B', 'C'] } })

B db.products.find({ category: { $ne: 'A' }, category: { $ne: 'B' }, category: { $ne: 'C' } })

C db.products.find({ category: { $not: { $in: ['A', 'B', 'C'] } } })

D db.products.find({ category: { $nin: ['A', 'B', 'C'] }, category: { $exists: true } })

💡 Hint

Consider how MongoDB uses indexes with $nin versus multiple $ne conditions.

Option A uses $nin directly, which can use the index efficiently. Option A uses multiple $ne conditions on the same field, which MongoDB cannot optimize well. Option A uses $not with $in, which is less efficient. Option A adds an unnecessary $exists check.

🔧 Debug

expert

Why does this $nin query return unexpected results?

A developer runs this query:
db.users.find({ roles: { $nin: ['admin', 'moderator'] } })
But it returns users who have 'admin' role. Why?

A Because 'roles' is an array field, $nin checks if the entire array is not in the list, not individual elements.

B Because $nin does not work on array fields at all.

C Because the query syntax is invalid and MongoDB ignores $nin.

D Because the roles field contains null values which match $nin.

💡 Hint

Think about how $nin behaves with array fields.

When the field is an array, $nin checks if the entire array value is not in the specified array. It does not check individual elements inside the array. To check if none of the array elements are in a set, use $not with $elemMatch or $nin inside $elemMatch.