$in for matching a set in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where a field matches any value in a set

Given a collection products with documents containing a category field, which query returns all products whose category is either 'electronics', 'books', or 'clothing'?

MongoDB

db.products.find({ category: { $in: ['electronics', 'books', 'clothing'] } })

A Returns products with category exactly matching the array ['electronics', 'books', 'clothing']

B Returns all products with category 'electronics', 'books', or 'clothing'

C Returns products with category not in 'electronics', 'books', or 'clothing'

D Returns all products regardless of category

💡 Hint

Think about how $in matches any value inside the array.

The $in operator matches documents where the field's value is any one of the values in the specified array. So it returns products whose category is 'electronics', 'books', or 'clothing'.

❓ query_result

intermediate

Using $in with numeric values

Consider a collection orders where each document has a numeric status_code. Which query returns orders with status codes 1, 3, or 5?

MongoDB

db.orders.find({ status_code: { $in: [1, 3, 5] } })

A Returns all orders regardless of status_code

B Returns orders with status_code exactly equal to the array [1, 3, 5]

C Returns orders with status_code 1, 3, or 5

D Returns orders with status_code not equal to 1, 3, or 5

💡 Hint

Remember $in checks if the field value is in the array.

The $in operator matches documents where the status_code is any of the values 1, 3, or 5.

📝 Syntax

advanced

Identify the syntax error in $in usage

Which of the following MongoDB queries has a syntax error when using $in to match the tags field?

A db.collection.find({ tags: { $in: 'red', 'blue', 'green' } })

B db.collection.find({ tags: { $in: ['red', 'blue', 'green'] } })

C )} } ]'neerg' ,'eulb' ,'der'[ :ni$ { :sgat {(dnif.noitcelloc.bd

D db.collection.find({ tags: { $in: ['red'] } })

💡 Hint

Check how the array is passed to $in.

The $in operator requires an array as its value. Option A incorrectly passes multiple string arguments instead of an array, causing a syntax error.

❓ optimization

advanced

Optimizing queries with $in on indexed fields

You have a large collection users with an index on the role field. Which query will best use the index to find users with roles 'admin', 'editor', or 'viewer'?

A db.users.find({ $or: [{ role: 'admin' }, { role: 'editor' }, { role: 'viewer' }] })

B db.users.find({ role: { $all: ['admin', 'editor', 'viewer'] } })

C db.users.find({ role: { $nin: ['admin', 'editor', 'viewer'] } })

D db.users.find({ role: { $in: ['admin', 'editor', 'viewer'] } })

💡 Hint

Think about how $in uses indexes compared to $or.

The $in operator efficiently uses indexes on the field to match any of the specified values. While $or can also use indexes, $in is more concise and optimized for this use case.

🧠 Conceptual

expert

Understanding $in behavior with arrays in documents

In a collection posts, each document has a field tags which is an array of strings. What does the query db.posts.find({ tags: { $in: ['mongodb', 'database'] } }) return?

A Documents where the tags array contains at least one of 'mongodb' or 'database'

B Documents where the tags array exactly matches ['mongodb', 'database']

C Documents where the tags array contains both 'mongodb' and 'database'

D Documents where the tags array contains neither 'mongodb' nor 'database'

💡 Hint

Remember how $in works with array fields in documents.

The $in operator matches if any element of the tags array is in the specified list. It does not require all elements to match or exact array equality.

Practice

1.

What does the $in operator do in MongoDB queries?

easy

A. Deletes documents matching a condition

B. Finds documents where a field is greater than a value

C. Updates documents by adding new fields

D. Finds documents where a field matches any value in a given list

Solution

1. Step 1: Understand the purpose of $in

   The $in operator is used to match documents where a field's value is equal to any value in a specified array.

2. Step 2: Compare with other options

   Options B, C, and D describe different MongoDB operations unrelated to $in.

3. Final Answer:

   Finds documents where a field matches any value in a given list -> Option D

4. Quick Check:

   $in matches any value in list [OK]

Hint: Remember: $in checks if field is in a list [OK]

Common Mistakes:

• Confusing $in with comparison operators like $gt
• Thinking $in updates or deletes documents
• Using $in without an array of values

2.

Which of the following is the correct syntax to find documents where the field color is either "red", "blue", or "green"?

{ color: { $in: ["red", "blue", "green"] } }

easy

A. { color: { $in: ["red", "blue", "green"] } }

B. { color: { $in: "red", "blue", "green" } }

C. { color: { $in: "red|blue|green" } }

D. { color: { $in: { "red", "blue", "green" } } }

Solution

1. Step 1: Check the correct use of $in syntax

   The $in operator requires an array of values inside square brackets []. { color: { $in: ["red", "blue", "green"] } } correctly uses an array with strings.

2. Step 2: Identify syntax errors in other options

   { color: { $in: "red", "blue", "green" } } misses brackets for the array. { color: { $in: "red|blue|green" } } uses a string with pipes instead of an array. { color: { $in: { "red", "blue", "green" } } } uses curly braces which is invalid for arrays.

3. Final Answer:

   { color: { $in: ["red", "blue", "green"] } } -> Option A

4. Quick Check:

   $in needs an array [OK]

Hint: Always use square brackets [] for $in values [OK]

Common Mistakes:

• Using strings instead of arrays for $in
• Using curly braces {} instead of square brackets []
• Separating values with commas but missing brackets

3.

Given the collection products with documents:

[{ "name": "Pen", "category": "stationery" }, { "name": "Apple", "category": "fruit" }, { "name": "Notebook", "category": "stationery" }, { "name": "Banana", "category": "fruit" }]

What will be the result of this query?

db.products.find({ category: { $in: ["fruit"] } })

medium

A. [{ "name": "Pen", "category": "stationery" }, { "name": "Notebook", "category": "stationery" }]

B. [{ "name": "Apple", "category": "fruit" }, { "name": "Banana", "category": "fruit" }]

C. []

D. All documents in the collection

Solution

1. Step 1: Understand the query filter

   The query filters documents where the category field is in the array ["fruit"]. This means only documents with category "fruit" will match.

2. Step 2: Identify matching documents

   Documents with "Apple" and "Banana" have category "fruit", so they are returned. Others are excluded.

3. Final Answer:

   [{ "name": "Apple", "category": "fruit" }, { "name": "Banana", "category": "fruit" }] -> Option B

4. Quick Check:

   Filter by category in ["fruit"] returns fruit items [OK]

Hint: Match only documents with field in the given list [OK]

Common Mistakes:

• Expecting documents with other categories to appear
• Confusing $in with $nin (not in)
• Thinking $in matches substrings instead of exact values

4.

What is wrong with this MongoDB query?

db.users.find({ age: { $in: 25, 30, 35 } })

medium

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

B. The field name age is invalid

C. The $in operator requires an array of values, not separate arguments

D. The query is correct and will return matching documents

Solution

1. Step 1: Check the syntax of $in usage

   The $in operator expects a single array argument containing values to match. Here, values are passed as separate arguments, which is invalid syntax.

2. Step 2: Identify the correct syntax

   The correct syntax is { age: { $in: [25, 30, 35] } } with square brackets around the values.

3. Final Answer:

   The $in operator requires an array of values, not separate arguments -> Option C

4. Quick Check:

   $in needs an array [OK]

Hint: Always wrap $in values in square brackets [OK]

Common Mistakes:

• Passing multiple values without an array
• Confusing $in with other operators
• Assuming query is valid without brackets

5.

You have a collection orders with documents containing a field status. You want to find all orders where the status is either "pending", "shipped", or "delivered" but exclude those with status "cancelled" or "returned". Which query correctly uses $in and $nin to achieve this?

hard

A. { $or: [ { status: { $in: ["pending", "shipped", "delivered"] } }, { status: { $nin: ["cancelled", "returned"] } } ] }

B. { status: { $in: ["pending", "shipped", "delivered"] }, status: { $nin: ["cancelled", "returned"] } }

C. { status: { $in: ["pending", "shipped", "delivered"] }, $nin: ["cancelled", "returned"] }

D. { status: { $in: ["pending", "shipped", "delivered"], $nin: ["cancelled", "returned"] } }

Solution

1. Step 1: Understand the need to combine $in and $nin on the same field

   We want documents where status is in one list but not in another. MongoDB does not allow combining $in and $nin inside the same object for a field. Instead, both conditions must be combined using an implicit AND by specifying both in the query object.

2. Step 2: Analyze each option's structure

   { status: { $in: ["pending", "shipped", "delivered"] }, status: { $nin: ["cancelled", "returned"] } } uses the same field key twice with different operators, which is invalid JSON syntax. { $or: [ { status: { $in: ["pending", "shipped", "delivered"] } }, { status: { $nin: ["cancelled", "returned"] } } ] } uses $or, which applies OR logic, not the required AND. { status: { $in: ["pending", "shipped", "delivered"] }, $nin: ["cancelled", "returned"] } places $nin outside the field, which is invalid. { status: { $in: ["pending", "shipped", "delivered"], $nin: ["cancelled", "returned"] } } tries to combine $in and $nin inside the same object, which is invalid in MongoDB.

3. Step 3: Correct approach

   The correct way is to combine both conditions using $and or by specifying both operators inside an object using $and or by using the implicit AND by combining conditions in the query object like: { status: { $in: [...] }, status: { $nin: [...] } } is invalid JSON because keys repeat. So the correct query is:
   { status: { $in: [...] }, status: { $nin: [...] } } is invalid.
   Instead, use:
   { status: { $in: [...] }, status: { $nin: [...] } } is invalid.
   So the correct way is:
   { status: { $in: [...] }, $nin: [...] } is invalid.
   Therefore, the correct query is:
   { status: { $in: [...] }, $nin: [...] } is invalid.
   Hence, the only valid way is to use $and operator:
   { $and: [ { status: { $in: [...] } }, { status: { $nin: [...] } } ] }

4. Final Answer:

   { $and: [ { status: { $in: ["pending", "shipped", "delivered"] } }, { status: { $nin: ["cancelled", "returned"] } } ] } -> Option A (modified)

5. Quick Check:

   Use $and to combine $in and $nin conditions [OK]

Hint: Use $and to combine multiple conditions on the same field [OK]

Common Mistakes:

• Repeating the same field key multiple times in query
• Placing $nin outside the field object
• Using separate objects for the same field without $and