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MongoDBquery~30 mins

$nin for not in set in MongoDB - Mini Project: Build & Apply

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$nin for not in set in MongoDB
📖 Scenario: You are managing a small online bookstore database. You want to find books that are not in certain genres to recommend to customers who want something different.
🎯 Goal: Build a MongoDB query using the $nin operator to find books whose genre is not in a given list of genres.
📋 What You'll Learn
Create a collection called books with specific book documents
Define a list of genres to exclude
Write a query using $nin to find books not in those genres
Complete the query to return only the book titles
💡 Why This Matters
🌍 Real World
Filtering data to exclude certain categories is common in recommendation systems, reporting, and data analysis.
💼 Career
Understanding $nin helps in writing flexible MongoDB queries for real-world applications like filtering products, users, or content.
Progress0 / 4 steps
1
Create the books collection with sample data
Create a MongoDB collection called books and insert these exact documents: { title: "The Hobbit", genre: "Fantasy" }, { title: "1984", genre: "Dystopian" }, { title: "To Kill a Mockingbird", genre: "Classic" }, { title: "The Great Gatsby", genre: "Classic" }, { title: "Neuromancer", genre: "Science Fiction" }.
MongoDB
Hint

Use db.books.insertMany([...]) with the exact documents listed.

2
Define the list of genres to exclude
Create a variable called excludedGenres and set it to an array containing the strings "Classic" and "Dystopian".
MongoDB
Hint

Use const excludedGenres = ["Classic", "Dystopian"] to define the array.

3
Write the query using $nin to find books not in excluded genres
Write a MongoDB query called query that finds documents in books where the genre field is $nin the excludedGenres array.
MongoDB
Hint

Use { genre: { $nin: excludedGenres } } to build the query object.

4
Complete the query to return only book titles
Write a MongoDB find command using query to find matching books in books and project only the title field (exclude _id). Save this to a variable called result.
MongoDB
Hint

Use db.books.find(query, { title: 1, _id: 0 }) to get only titles.

Practice

(1/5)
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