$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.

Practice

1. What does the MongoDB operator $nin do in a query?

easy

A. Selects documents where the field's value is in the specified array

B. Selects documents where the field's value is NOT in the specified array

C. Updates documents with values in the specified array

D. Deletes documents where the field's value is in the specified array

Solution

1. Step 1: Understand the purpose of $nin

   The $nin operator is used to filter documents where a field's value is NOT included in a given list of values.

2. Step 2: Compare with other operators

   Unlike $in which selects values inside the array, $nin excludes those values.

3. Final Answer:

   Selects documents where the field's value is NOT in the specified array -> Option B

4. Quick Check:

   $nin excludes values = B [OK]

Hint: Remember: $nin means NOT in list [OK]

Common Mistakes:

• Confusing $nin with $in
• Thinking it updates or deletes documents
• Using it to select values inside the array

2. Which of the following is the correct syntax to find documents where the field status is NOT 'active' or 'pending' using $nin?

easy

A. { status: { $nin: ['active', 'pending'] } }

B. { status: { $nin: 'active', 'pending' } }

C. { status: { $nin: ['active'] || ['pending'] } }

D. { status: { $nin: 'active' && 'pending' } }

Solution

1. Step 1: Check the correct $nin syntax

   The $nin operator requires an array of values inside square brackets to specify the excluded set.

2. Step 2: Validate each option

   { status: { $nin: ['active', 'pending'] } } correctly uses an array with two strings. Options B, C, and D have syntax errors or invalid expressions.

3. Final Answer:

   { status: { $nin: ['active', 'pending'] } } -> Option A

4. Quick Check:

   Correct array syntax for $nin = A [OK]

Hint: Use square brackets for arrays in $nin [OK]

Common Mistakes:

• Using multiple arguments instead of an array
• Using logical operators inside $nin
• Missing square brackets around values

3. Given the collection products with documents:

{ name: 'Pen', category: 'stationery' }

{ name: 'Apple', category: 'fruit' }

{ name: 'Notebook', category: 'stationery' }

{ name: 'Carrot', category: 'vegetable' }

What will be the result of this query?

db.products.find({ category: { $nin: ['fruit', 'vegetable'] } })

medium

A. All documents returned

B. [{ name: 'Apple', category: 'fruit' }, { name: 'Carrot', category: 'vegetable' }]

C. [] (empty array)

D. [{ name: 'Pen', category: 'stationery' }, { name: 'Notebook', category: 'stationery' }]

Solution

1. Step 1: Understand the $nin filter

   The query excludes documents where category is 'fruit' or 'vegetable'.

2. Step 2: Identify matching documents

   Documents with category 'stationery' are not in ['fruit', 'vegetable'], so they match. 'Pen' and 'Notebook' have 'stationery'.

3. Final Answer:

   [{ name: 'Pen', category: 'stationery' }, { name: 'Notebook', category: 'stationery' }] -> Option D

4. Quick Check:

   Exclude 'fruit' and 'vegetable' = stationery items only [OK]

Hint: Exclude unwanted categories with $nin array [OK]

Common Mistakes:

• Confusing $nin with $in
• Expecting documents with excluded categories
• Assuming empty result when some match

4. You wrote this query to exclude users with roles 'admin' or 'moderator':

db.users.find({ role: { $nin: 'admin', 'moderator' } })

But it throws an error. What is the problem?

medium

A. The $nin operator requires an array, not multiple arguments

B. The field name role is misspelled

C. The query should use $in instead of $nin

D. MongoDB does not support $nin operator

Solution

1. Step 1: Check $nin syntax

   $nin expects a single array argument listing values to exclude.

2. Step 2: Identify the error in the query

   The query passes two separate string arguments instead of one array, causing syntax error.

3. Final Answer:

   The $nin operator requires an array, not multiple arguments -> Option A

4. Quick Check:

   $nin needs array input = C [OK]

Hint: Always wrap values in an array for $nin [OK]

Common Mistakes:

• Passing multiple arguments instead of an array
• Using wrong operator $in by mistake
• Assuming $nin is unsupported

5. You have a collection orders with documents:

{ orderId: 1, status: 'shipped' }

{ orderId: 2, status: 'pending' }

{ orderId: 3, status: 'cancelled' }

{ orderId: 4, status: 'delivered' }

You want to find orders NOT in statuses 'pending' or 'cancelled' AND exclude orders with orderId 4. Which query correctly uses $nin to achieve this?

hard

A. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $ne: 4 } }

B. { status: { $nin: ['pending', 'cancelled', 4] } }

C. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $nin: [4] } }

D. { status: { $nin: ['pending', 'cancelled'] }, orderId: { $in: [4] } }

Solution

1. Step 1: Exclude statuses 'pending' and 'cancelled'

   Use { status: { $nin: ['pending', 'cancelled'] } } to exclude these statuses.

2. Step 2: Exclude orderId 4 using $nin

   Use { orderId: { $nin: [4] } } to exclude orderId 4 as well.

3. Step 3: Combine conditions

   Both conditions together filter out unwanted statuses and orderId 4.

4. Final Answer:

   { status: { $nin: ['pending', 'cancelled'] }, orderId: { $nin: [4] } } -> Option C

5. Quick Check:

   Use $nin on both fields = A [OK]

Hint: Use separate $nin for each field to exclude multiple sets [OK]

Common Mistakes:

• Putting orderId inside status array
• Using $ne instead of $nin for multiple values
• Using $in to exclude values