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.

Practice

1. Which MongoDB query syntax correctly accesses a nested field named address.city inside a document?

easy

A. { address.city() : "New York" }

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

C. { "address->city": "New York" }

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

Solution

1. Step 1: Understand dot notation for nested fields

   MongoDB uses dot notation like "address.city" to access nested fields inside documents.

2. Step 2: Identify correct query syntax

   The correct query uses a string key with dot notation: { "address.city": "New York" }.

3. Final Answer:

   { "address.city": "New York" } -> Option D

4. Quick Check:

   Dot notation = { "address.city": value } [OK]

Hint: Use quotes and dot notation to access nested fields [OK]

Common Mistakes:

• Using nested objects instead of dot notation in query
• Using arrow or parentheses instead of dot notation
• Not quoting the nested field path

2. Which of the following is the correct MongoDB query to find documents where the nested field profile.details.age equals 30?

easy

A. { profile.details.age = 30 }

B. { "profile.details.age": 30 }

C. { profile.details.age: 30 }

D. { 'profile.details.age' == 30 }

Solution

1. Step 1: Use string keys with dot notation in MongoDB queries

   MongoDB requires the nested field path as a string key with dots, like "profile.details.age".

2. Step 2: Use colon for key-value pairs in query objects

   The correct syntax uses colon (:), not equals (=) or double equals (==), so { "profile.details.age": 30 } is correct.

3. Final Answer:

   { "profile.details.age": 30 } -> Option B

4. Quick Check:

   Dot notation with colon = correct query [OK]

Hint: Use colon and quotes for nested keys in queries [OK]

Common Mistakes:

• Using = or == instead of colon in query
• Not quoting the nested field path
• Using object notation instead of dot notation

3. Given the collection documents:

{ "user": { "contact": { "email": "[e-mail]" } } }, { "user": { "contact": { "email": "[e-mail]" } } }, { "user": { "contact": { "phone": "12345" } } }

What will the query db.collection.find({ "user.contact.email": { $exists: true } }) return?

medium

A. Documents where user.contact has any field

B. Documents where user.contact.phone exists

C. Documents where user.contact.email exists

D. All documents in the collection

Solution

1. Step 1: Understand $exists operator with nested fields

   The query checks if the nested field "user.contact.email" exists in documents.

2. Step 2: Identify which documents have that nested field

   Only the first two documents have "user.contact.email"; the third has "user.contact.phone" instead.

3. Final Answer:

   Documents where user.contact.email exists -> Option C

4. Quick Check:

   $exists true filters documents with that nested field [OK]

Hint: Use $exists to check nested field presence [OK]

Common Mistakes:

• Assuming $exists checks parent fields
• Confusing email and phone fields
• Expecting all documents to match

4. You wrote this query to find documents where profile.address.zipcode is "12345":

db.users.find({ profile.address.zipcode: "12345" })

But it gives a syntax error. What is the fix?

medium

A. Use quotes around the nested field: { "profile.address.zipcode": "12345" }

B. Replace dots with underscores: { profile_address_zipcode: "12345" }

C. Use double equals: { "profile.address.zipcode" == "12345" }

D. Remove the nested field and query only { zipcode: "12345" }

Solution

1. Step 1: Identify syntax error cause

   MongoDB requires string keys with dot notation quoted in queries to avoid syntax errors.

2. Step 2: Correct query syntax

   Wrap the nested field path in quotes: { "profile.address.zipcode": "12345" } fixes the syntax error.

3. Final Answer:

   Use quotes around the nested field: { "profile.address.zipcode": "12345" } -> Option A

4. Quick Check:

   Quotes fix dot notation syntax errors [OK]

Hint: Always quote nested field keys in queries [OK]

Common Mistakes:

• Not quoting nested field keys
• Using == instead of colon
• Replacing dots with underscores incorrectly

5. You have documents with deeply nested fields like settings.preferences.notifications.email.enabled. How would you write a MongoDB query to find all documents where email notifications are enabled (true), regardless of nesting depth?

hard

A. { "settings.preferences.notifications.email.enabled": true }

B. { settings: { preferences: { notifications: { email: { enabled: true } } } } }

C. { "email.enabled": true }

D. { "settings.preferences.notifications.email.enabled": { $eq: "true" } }

Solution

1. Step 1: Use dot notation string to access deeply nested field

   MongoDB queries use dot notation strings to reach any depth, so "settings.preferences.notifications.email.enabled" is correct.

2. Step 2: Match boolean value correctly

   Use true (boolean) without quotes to match enabled field, so { "settings.preferences.notifications.email.enabled": true } is correct.

3. Final Answer:

   { "settings.preferences.notifications.email.enabled": true } -> Option A

4. Quick Check:

   Dot notation + boolean value = correct query [OK]

Hint: Use full dot path with boolean true (no quotes) [OK]

Common Mistakes:

• Using nested objects instead of dot notation
• Quoting boolean true as string
• Using partial nested paths