One-to-many referencing pattern in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find all orders for a specific customer

Given two collections, customers and orders, where orders references customers by customer_id, which query returns all orders for the customer with _id equal to 123?

A db.orders.find({ _id: 123 })

B db.customers.find({ _id: 123 }).orders

C db.orders.find({ customer_id: 123 })

D db.orders.aggregate([{ $match: { _id: 123 } }])

💡 Hint

Think about which collection stores the orders and how they link to customers.

Orders reference customers by customer_id, so to find orders for customer 123, filter orders where customer_id is 123.

📝 Syntax

intermediate

Correct syntax for referencing in MongoDB

Which of the following is the correct way to store a one-to-many reference from a customer document to multiple order documents using ObjectId references?

A { _id: ObjectId('abc'), orders: { o1: ObjectId('o1'), o2: ObjectId('o2') } }

B { _id: ObjectId('abc'), orders: 'o1,o2' }

C { _id: ObjectId('abc'), orders: ObjectId('o1') }

D { _id: ObjectId('abc'), orders: [ObjectId('o1'), ObjectId('o2')] }

💡 Hint

Think about how to store multiple references in an array.

An array of ObjectIds correctly represents multiple references; options A and D use wrong types, C only stores one reference.

❓ optimization

advanced

Efficiently retrieving customer with their orders

You want to retrieve a customer document along with all their orders in one query. Which MongoDB aggregation pipeline stage should you use to join customers and orders collections on _id and customer_id?

A { $group: { _id: '$customer_id', orders: { $push: '$$ROOT' } } }

B { $lookup: { from: 'orders', localField: '_id', foreignField: 'customer_id', as: 'orders' } }

C { $match: { customer_id: '_id' } }

D { $project: { orders: 1 } }

💡 Hint

Look for the stage that performs a left outer join.

$lookup performs a join between collections; other options do not join or filter correctly.

🔧 Debug

advanced

Why does this query return no orders?

A developer runs this query to find orders for customer with _id 'abc123': db.orders.find({ customer_id: 'abc123' }). But it returns no results, even though orders exist. What is the likely cause?

A customer_id is stored as ObjectId, but query uses string 'abc123'

B The orders collection is empty

C The query syntax is invalid

D The customer _id is incorrect

💡 Hint

Check the data types of the fields used in the query.

MongoDB matches types strictly; querying with a string won't match ObjectId fields.

🧠 Conceptual

expert

Best practice for one-to-many referencing in MongoDB

In a one-to-many relationship where a customer can have thousands of orders, what is the best practice for storing references in MongoDB?

A Store customer ObjectId inside each order document and query orders by customer_id

B Store an array of all order ObjectIds inside the customer document

C Embed all order documents inside the customer document

D Duplicate customer data inside each order document

💡 Hint

Consider document size limits and query efficiency.

Storing customer_id in orders avoids large arrays in customer documents and supports efficient queries.