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.