Dot notation for embedded documents in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find the city of the first user

Given the collection users where each document has an embedded address field with a city subfield, what is the output of this query?

db.users.findOne({}, {"address.city": 1, _id: 0})

MongoDB

db.users.insertMany([
  {name: "Alice", address: {city: "New York", zip: "10001"}},
  {name: "Bob", address: {city: "Los Angeles", zip: "90001"}}
])

A {"address": "New York"}

B {"address": {"city": "New York"}}

C {"city": "New York"}

D {"address.city": "New York"}

💡 Hint

Remember that dot notation in projection returns the embedded document with only the specified field.

The projection {"address.city": 1, _id: 0} returns the address field but only includes the city subfield. So the output is {"address": {"city": "New York"}}.

❓ query_result

intermediate

Retrieve nested field using dot notation

Consider a collection orders with documents like:

{
  _id: 1,
  customer: {name: "John", contact: {email: "[e-mail]", phone: "123-456"}},
  total: 100
}

What does this query return?

db.orders.findOne({}, {"customer.contact.email": 1, _id: 0})

MongoDB

db.orders.insertOne({
  _id: 1,
  customer: {name: "John", contact: {email: "[e-mail]", phone: "123-456"}},
  total: 100
})

A {"contact": {"email": "[e-mail]"}}

B {"customer.contact.email": "[e-mail]"}

C {"email": "[e-mail]"}

D {"customer": {"contact": {"email": "[e-mail]"}}}

💡 Hint

Projection with dot notation returns the full path of embedded documents with only the specified leaf field.

The projection includes the full path customer.contact but only the email field inside contact, so the output nests customer and contact with only email.

📝 Syntax

advanced

Identify the invalid dot notation query

Which of the following MongoDB queries using dot notation will cause a syntax error or fail to run?

A db.products.find({}, {"details.price": 1})

B db.products.find({}, {"details.price": 1, _id: 0})

C db.products.find({}, {details.price: 1, _id: 0})

D )}0 :di_ ,1 :"ecirp.sliated"{ ,}{(dnif.stcudorp.bd

💡 Hint

Check how keys with dots must be quoted in JavaScript objects.

Option C is invalid because keys with dots must be quoted as strings in the projection object. Without quotes, it causes a syntax error.

❓ optimization

advanced

Optimize query to return only nested field values

You want to retrieve only the score field inside the embedded stats document from all documents in players. Which query is the most efficient to return only the score values without extra nesting?

A db.players.aggregate([{ $project: { score: "$stats.score", _id: 0 } }])

B db.players.find({}, {score: "$stats.score", _id: 0})

C db.players.find({}, {score: 1, _id: 0})

D db.players.find({}, {"stats.score": 1, _id: 0})

💡 Hint

Projection with dot notation returns nested documents; aggregation can reshape fields.

Option A uses aggregation to project the nested field as a top-level field, returning only the score values without nesting inside stats.

🧠 Conceptual

expert

Understanding dot notation behavior with arrays

Given a collection books with documents like:

{
  title: "Book A",
  authors: [
    {name: "Alice", age: 30},
    {name: "Bob", age: 40}
  ]
}

What does the query db.books.findOne({}, {"authors.name": 1, _id: 0}) return?

A {"authors": [{"name": "Alice"}, {"name": "Bob"}]}

B {"authors.name": ["Alice", "Bob"]}

C {"name": ["Alice", "Bob"]}

D {"authors": "Alice, Bob"}

💡 Hint

Projection with dot notation on arrays returns the array with only the specified fields in each element.

The projection returns the authors array but each element only includes the name field, preserving the array structure.