Sorting by multiple fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Sorting documents by age ascending then name descending

Given a collection people with documents containing name and age, what is the output of this query sorting by age ascending and name descending?

MongoDB

db.people.find().sort({age: 1, name: -1})

A [{name: "Zara", age: 25}, {name: "Anna", age: 25}, {name: "Bob", age: 30}]

B [{name: "Bob", age: 30}, {name: "Anna", age: 25}, {name: "Zara", age: 25}]

C [{name: "Anna", age: 25}, {name: "Zara", age: 25}, {name: "Bob", age: 30}]

D [{name: "Anna", age: 25}, {name: "Bob", age: 30}, {name: "Zara", age: 25}]

💡 Hint

Remember ascending means smallest to largest, descending means reverse alphabetical order.

The query sorts first by age ascending (25 before 30), then by name descending (Zara before Anna for age 25).

📝 Syntax

intermediate

Identify the syntax error in sorting by multiple fields

Which option contains a syntax error in the MongoDB sort method for sorting by score descending and date ascending?

MongoDB

db.records.find().sort({score: -1, date: 1})

A db.records.find().sort({score: -1, date: 1})

B db.records.find().sort({score: -1; date: 1})

C )}1 :etad ,1- :erocs{(tros.)(dnif.sdrocer.bd

D db.records.find().sort({score: -1 date: 1})

💡 Hint

Check the punctuation between fields inside the sort object.

Option B uses a semicolon instead of a comma between fields, which is invalid syntax in JavaScript objects.

❓ optimization

advanced

Optimizing sorting on multiple fields with indexes

Which index will optimize the query db.orders.find().sort({customerId: 1, orderDate: -1}) best?

A An index on {orderDate: -1, customerId: 1}

B An index on {orderDate: 1, customerId: 1}

C An index on {customerId: 1, orderDate: -1}

D An index on {customerId: -1, orderDate: 1}

💡 Hint

Indexes should match the sort fields and their order/direction.

The index must start with customerId ascending and then orderDate descending to match the sort exactly.

🔧 Debug

advanced

Why does this multi-field sort not work as expected?

A developer runs db.products.find().sort({category: 1, price: 1}) but the results are not sorted by price within each category. What is the likely cause?

A The collection has no index on category and price, causing inefficient sorting.

B The sort object keys are unordered, so MongoDB ignores the second field.

C MongoDB sorts only by the first field in the sort object, ignoring others.

D The query needs a projection to include category and price for sorting.

💡 Hint

Think about how MongoDB uses indexes to optimize sorting.

Without a supporting index, MongoDB may perform an in-memory sort that can be limited or inefficient, causing unexpected order.

🧠 Conceptual

expert

Understanding sort order precedence in MongoDB

If you run db.logs.find().sort({level: -1, timestamp: 1}), which statement best describes the sorting behavior?

A Documents are sorted randomly because mixed sort directions are not allowed.

B Documents are sorted by timestamp ascending only, ignoring level.

C Documents are sorted by level ascending, then timestamp descending.

D Documents are sorted first by level descending, then by timestamp ascending within each level.

💡 Hint

The order of fields in the sort object defines precedence.

MongoDB sorts by the first field (level descending), then by the second field (timestamp ascending) within groups of the first.