Document model mental model (JSON/BSON) in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB find query?

Consider a collection users with documents like:

{ "name": "Alice", "age": 30, "hobbies": ["reading", "hiking"] }

What will this query return?

db.users.find({ "hobbies": "hiking" })

MongoDB

db.users.find({ "hobbies": "hiking" })

A [{ "name": "Alice", "age": 30, "hobbies": ["reading", "hiking"] }]

B []

C [{ "name": "Alice", "age": 30 }]

D [{ "hobbies": "hiking" }]

💡 Hint

Think about how MongoDB matches array fields with a value.

MongoDB matches documents where the array contains the value directly. So it returns the full document where hobbies include "hiking".

🧠 Conceptual

intermediate

Which BSON data type is used to store a date?

In MongoDB's BSON format, which data type stores date and time values?

A Timestamp

B ObjectId

C Date

D String

💡 Hint

It is a special BSON type different from a string.

The BSON Date type stores date and time as milliseconds since the Unix epoch.

📝 Syntax

advanced

Which option correctly inserts a nested document in MongoDB?

You want to insert a document with a nested address field. Which command is correct?

MongoDB

db.customers.insertOne({ name: "John", address: { city: "NY", zip: "10001" } })

A db.customers.insertOne({ name: "John", address: { city: "NY", zip: "10001" } })

B db.customers.insertOne({ name: "John", address: [ city: "NY", zip: "10001" ] })

C db.customers.insertOne({ name: "John", address: ( city: "NY", zip: "10001" ) })

D db.customers.insertOne({ name: "John", address: "city: NY, zip: 10001" })

💡 Hint

Remember nested documents use curly braces, not brackets or parentheses.

Nested documents are represented as objects with curly braces. Brackets are for arrays, parentheses are invalid here.

❓ optimization

advanced

Which index type improves search on nested fields in MongoDB?

You want to speed up queries filtering by a nested field address.city. Which index type should you create?

A Create a multikey index on <code>address.city</code>

B Create a compound index on <code>address.city</code> and <code>address.zip</code>

C Create a single field index on <code>address</code>

D Create a single field index on <code>address.city</code>

💡 Hint

Think about indexing a specific nested field.

Creating a single field index on the nested field address.city allows efficient queries filtering by city.

🔧 Debug

expert

Why does this MongoDB query return no results?

Given documents with a field tags as an array of strings, why does this query return no documents?

db.posts.find({ tags: { $eq: ["mongodb", "database"] } })

A Because $eq does not match arrays, only single values

B Because the query looks for exact array match, but documents have tags in different order

C Because $eq requires a nested query with $all for arrays

D Because the field name is incorrect; it should be 'tag' not 'tags'

💡 Hint

Think about how MongoDB compares arrays in queries.

The $eq operator matches the entire array exactly, including order. If the stored array order differs, no match occurs.

Practice

1. Which of the following best describes a MongoDB document?

easy

A. A compiled program file

B. A table with rows and columns like in SQL

C. A set of key-value pairs similar to a JSON object

D. A flat file storing plain text data

Solution

1. Step 1: Understand MongoDB document structure

   MongoDB stores data as documents, which are collections of key-value pairs similar to JSON objects.

2. Step 2: Compare with other data formats

   Unlike tables or flat files, documents can store nested data and arrays, making them flexible and structured.

3. Final Answer:

   A set of key-value pairs similar to a JSON object -> Option C

4. Quick Check:

   Document = JSON-like key-value pairs [OK]

Hint: Think JSON object when you hear MongoDB document [OK]

Common Mistakes:

• Confusing documents with SQL tables
• Thinking documents are flat text files
• Assuming documents are executable files

2. Which of the following is the correct way to represent a nested document in MongoDB?

easy

A. { "name": "Alice", "address": { "city": "NY", "zip": 10001 } }

B. { "name": "Alice", "address": "city: NY, zip: 10001" }

C. { "name": "Alice", "address": ["city", "NY", "zip", 10001] }

D. { "name": "Alice", "address": ("city": "NY", "zip": 10001) }

Solution

1. Step 1: Identify correct JSON syntax for nested documents

   Nested documents are represented as objects inside another object using curly braces {} with key-value pairs.

2. Step 2: Check each option's syntax

   { "name": "Alice", "address": { "city": "NY", "zip": 10001 } } uses proper JSON syntax with nested braces. The other options use incorrect formats like strings, arrays, or parentheses.

3. Final Answer:

   { "name": "Alice", "address": { "city": "NY", "zip": 10001 } } -> Option A

4. Quick Check:

   Nested document = object inside object with braces [OK]

Hint: Nested documents use curly braces inside braces [OK]

Common Mistakes:

• Using strings instead of nested objects
• Using arrays for nested key-value pairs
• Using parentheses instead of braces

3. Given the document { "name": "Bob", "scores": [85, 90, 78] }, what is the value of the scores field?

medium

A. "85, 90, 78"

B. [85, 90, 78]

C. {85: true, 90: true, 78: true}

D. 85

Solution

1. Step 1: Identify the data type of the scores field

   The scores field contains square brackets [], which represent an array in JSON/BSON.

2. Step 2: Understand array representation

   The array holds the numbers 85, 90, and 78 as elements, so the value is the list [85, 90, 78].

3. Final Answer:

   [85, 90, 78] -> Option B

4. Quick Check:

   Square brackets = array = [85, 90, 78] [OK]

Hint: Square brackets mean array, not string or object [OK]

Common Mistakes:

• Confusing array with string
• Thinking array is a key-value object
• Selecting only one element instead of full array

4. Identify the error in this MongoDB document: { "title": "Book", "pages": "300", "author": { "name": "John", "age": 45 }

medium

A. Missing closing brace for the document

B. Pages field should be a number, not a string

C. Title field cannot be a string

D. Author name must be an array

Solution

1. Step 1: Check document syntax carefully

   The document starts with { but does not have a matching closing brace } at the end.

2. Step 2: Validate other fields

   While pages is a string, MongoDB allows strings for numbers; title as string is valid; author.name as string is valid.

3. Final Answer:

   Missing closing brace for the document -> Option A

4. Quick Check:

   Every { must have matching } [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Ignoring missing braces
• Thinking string numbers are invalid
• Assuming arrays are required for nested objects

5. You want to store a product with multiple colors and a supplier's contact info inside one MongoDB document. Which structure correctly models this?

hard

A. { "product": "Shirt", "colors": "red, blue", "supplier": "ABC Co, 123-456" }

B. { "product": "Shirt", "colors": ("red", "blue"), "supplier": { "name": "ABC Co", "phone": 123456 } }

C. { "product": "Shirt", "colors": { "red": true, "blue": true }, "supplier": ["ABC Co", "123-456"] }

D. { "product": "Shirt", "colors": ["red", "blue"], "supplier": { "name": "ABC Co", "phone": "123-456" } }

Solution

1. Step 1: Understand how to store multiple values and nested info

   Multiple colors should be stored as an array, and supplier info as a nested document with key-value pairs.

2. Step 2: Evaluate each option's structure

   { "product": "Shirt", "colors": ["red", "blue"], "supplier": { "name": "ABC Co", "phone": "123-456" } } correctly uses an array for colors and a nested document for supplier. { "product": "Shirt", "colors": "red, blue", "supplier": "ABC Co, 123-456" } uses strings instead of structured data. { "product": "Shirt", "colors": { "red": true, "blue": true }, "supplier": ["ABC Co", "123-456"] } uses an object for colors incorrectly and an array for supplier. { "product": "Shirt", "colors": ("red", "blue"), "supplier": { "name": "ABC Co", "phone": 123456 } } uses parentheses which are invalid in JSON.

3. Final Answer:

   { "product": "Shirt", "colors": ["red", "blue"], "supplier": { "name": "ABC Co", "phone": "123-456" } } -> Option D

4. Quick Check:

   Arrays for lists, objects for nested info [OK]

Hint: Use arrays for lists, objects for nested details [OK]

Common Mistakes:

• Using strings instead of arrays for multiple values
• Using invalid parentheses instead of braces
• Confusing arrays and objects for nested data