Querying nested fields at any depth in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with a nested field value

Given a collection users where each document may have a nested field profile.address.city, which query returns all users living in 'New York'?

A { 'profile.city': 'New York' }

B { 'profile.address.city': 'New York' }

C { 'address.city': 'New York' }

D { 'profile.address': 'New York' }

💡 Hint

Use dot notation to access nested fields in MongoDB queries.

MongoDB uses dot notation to query nested fields. To find documents where the city inside address inside profile is 'New York', you must specify the full path 'profile.address.city'.

❓ query_result

intermediate

Query nested arrays with dot notation

In a collection orders, each document has an array field items where each item has a nested field product.details.category. Which query finds orders containing at least one item in the 'electronics' category?

A { 'items.product.category': 'electronics' }

B { 'items.category': 'electronics' }

C { 'product.details.category': 'electronics' }

D { 'items.product.details.category': 'electronics' }

💡 Hint

Dot notation works inside arrays to match nested fields in array elements.

MongoDB matches array elements when you use dot notation. Querying 'items.product.details.category' matches any item in the items array whose product.details.category equals 'electronics'.

📝 Syntax

advanced

Identify the syntax error in nested field query

Which of the following MongoDB queries will cause a syntax error when trying to find documents with nested field info.stats.score greater than 50?

A { 'info.stats.score': { $gt: 50 } }

B { 'info.stats.score': { '$gt': 50 } }

C { info.stats.score: { $gt: 50 } }

D } } 05 :tg$ { :'erocs.stats.ofni' {

💡 Hint

Field names with dots must be quoted as strings in MongoDB queries.

In option C, the field name info.stats.score is not quoted as a string, which is invalid syntax in JavaScript object keys when they contain dots. This causes a syntax error.

❓ optimization

advanced

Optimize query on deeply nested fields

You have a large collection logs with documents containing nested field event.details.user.id. Which index will optimize queries filtering by this nested user id?

A { 'event.details.user.id': 1 }

B { 'event.user.id': 1 }

C { 'details.user.id': 1 }

D { 'user.id': 1 }

💡 Hint

Indexes must match the exact field path used in queries.

To optimize queries filtering on 'event.details.user.id', the index must be created on the exact nested field path 'event.details.user.id'. Other paths do not match and won't optimize the query.

🔧 Debug

expert

Why does this nested field query return no results?

A developer runs this query on collection profiles:
db.profiles.find({ 'contact.phone.mobile': { $exists: true } })
But it returns no documents, even though some documents have a mobile phone number under contact.phone.mobile. What is the most likely reason?

MongoDB

db.profiles.find({ 'contact.phone.mobile': { $exists: true } })

A Some documents have <code>contact.phone</code> as an array, so dot notation does not match nested fields inside array elements.

B The field <code>contact.phone.mobile</code> is misspelled; it should be <code>contact.mobile.phone</code>.

C The query syntax is invalid because $exists cannot be used with nested fields.

D The collection <code>profiles</code> is empty, so no documents are returned.

💡 Hint

Dot notation does not traverse inside arrays automatically.

If 'contact.phone' is an array, querying 'contact.phone.mobile' does not match nested fields inside array elements. You must use $elemMatch or other operators to query nested fields inside arrays.