Querying array elements directly in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with a specific array element

Given a collection products where each document has an array field tags, which query returns all documents where tags contains the string "organic"?

MongoDB

db.products.find({tags: "organic"})

A db.products.find({tags: {$elemMatch: { $eq: "organic" }}})

B db.products.find({tags: "organic"})

C db.products.find({tags: {$in: ["organic"]}})

D db.products.find({tags: {$all: ["organic"]}})

💡 Hint

Check how MongoDB matches array elements directly by value.

In MongoDB, querying an array field with a value directly matches documents where the array contains that value. So {tags: "organic"} finds documents where tags includes "organic".

Other operators like $elemMatch, $in, and $all have different purposes and are not needed here.

❓ query_result

intermediate

Query documents with array elements matching a condition

Given a collection orders where each document has an array items with subdocuments containing price, which query returns documents where at least one item has a price greater than 100?

MongoDB

db.orders.find({"items.price": {$gt: 100}})

A db.orders.find({items: {$elemMatch: {price: {$gt: 100}}}})

B db.orders.find({items: {$all: [{price: {$gt: 100}}]}})

C db.orders.find({items: {price: {$gt: 100}}})

D db.orders.find({"items.price": {$gt: 100}})

💡 Hint

Use dot notation to query array subdocuments directly.

Using "items.price": {$gt: 100} queries documents where any element in the items array has a price greater than 100.

Option D also works but is more verbose. Option D is invalid because items is an array, so matching a subdocument directly without $elemMatch or dot notation won't work. Option D is incorrect because $all expects exact matches.

📝 Syntax

advanced

Identify the syntax error in querying array elements

Which query will cause a syntax error when trying to find documents where the array scores contains a value less than 50?

A db.students.find({scores: {$lt 50}})

B db.students.find({scores: {$elemMatch: {$lt: 50}}})

C db.students.find({scores: {$lt: 50}})

D db.students.find({"scores": {$lt: 50}})

💡 Hint

Check the syntax of the comparison operator inside the query object.

Option A is missing a colon between $lt and 50, causing a syntax error.

Options A, B, and C are valid MongoDB queries for this purpose.

❓ optimization

advanced

Optimize query for array element existence

You want to find documents in users where the array roles contains the value "admin". Which query is the most efficient?

A db.users.find({roles: "admin"})

B db.users.find({roles: {$elemMatch: {$eq: "admin"}}})

C db.users.find({roles: {$all: ["admin"]}})

D db.users.find({roles: {$in: ["admin"]}})

💡 Hint

Direct matching on array elements is usually fastest.

Querying with {roles: "admin"} directly matches documents where the array contains "admin" and is the simplest and most efficient.

Using $elemMatch or $in adds unnecessary complexity and overhead.

🧠 Conceptual

expert

Understanding array element matching behavior

Consider a collection events where each document has an array participants with subdocuments containing name and age. Which query returns documents where there is a participant named "Alice" who is exactly 30 years old?

A db.events.find({"participants.name": "Alice", "participants.age": 30})

B db.events.find({participants: {name: "Alice", age: 30}})

C db.events.find({participants: {$elemMatch: {name: "Alice", age: 30}}})

D db.events.find({participants: {$all: [{name: "Alice"}, {age: 30}]}})

💡 Hint

Use $elemMatch to match multiple conditions on the same array element.

Option C uses $elemMatch to ensure both conditions apply to the same participant object.

Option C tries to match the entire array element exactly, which is unlikely to match unless the object is exactly {name: "Alice", age: 30} with no extra fields.

Option C matches documents where any participant has name "Alice" and any participant (not necessarily the same) has age 30, which is not guaranteed to be the same participant.

Option C requires the array to contain both objects separately, which is not the intended meaning.