$lookup for joining collections in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a $lookup that joins the 'orders' collection with the 'customers' collection on the 'customerId' field.

MongoDB

db.orders.aggregate([{ $lookup: { from: "[1]", localField: "customerId", foreignField: "_id", as: "customerInfo" } }])

Drag options to blanks, or click blank then click option'

Acustomers

Bproducts

Csales

Demployees

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong collection name in the 'from' field.

Confusing 'from' with 'localField' or 'foreignField'.

The from field in $lookup specifies the collection to join with. Here, we join 'orders' with 'customers'.

2fill in blank

medium

Complete the code to join 'orders' with 'products' collection using $lookup on 'productId' and '_id'.

MongoDB

db.orders.aggregate([{ $lookup: { from: "products", localField: "[1]", foreignField: "_id", as: "productDetails" } }])

Drag options to blanks, or click blank then click option'

AcustomerId

BorderId

CproductId

DsupplierId

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'customerId' instead of 'productId' for product join.

Mixing up localField and foreignField.

The localField is the field in the 'orders' collection that matches the '_id' in 'products'. Here, it's 'productId'.

3fill in blank

hard

Fix the error in the $lookup stage to correctly join 'orders' with 'suppliers' on 'supplierId'.

MongoDB

db.orders.aggregate([{ $lookup: { from: "suppliers", localField: "supplierId", foreignField: "[1]", as: "supplierInfo" } }])

Drag options to blanks, or click blank then click option'

AsupplierId

BsupplierID

Cid

D_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'supplierId' as foreignField instead of '_id'.

Incorrect casing like 'supplierID'.

The foreignField should be the field in 'suppliers' that matches 'supplierId' in 'orders'. Usually, this is '_id'.

4fill in blank

hard

Fill both blanks to join 'orders' with 'employees' collection on 'employeeId' and project only 'orderId' and 'employeeName'.

MongoDB

db.orders.aggregate([ { $lookup: { from: "[1]", localField: "employeeId", foreignField: "_id", as: "employeeInfo" } }, { $project: { orderId: 1, [2]: "$employeeInfo.name" } } ])

Drag options to blanks, or click blank then click option'

Aemployees

BcustomerName

CemployeeName

Dorders

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong collection name in 'from'.

Projecting with incorrect field names.

The from collection is 'employees'. The projected field name for the employee's name is 'employeeName'.

5fill in blank

hard

Fill all three blanks to join 'orders' with 'categories' on 'categoryId', unwind the joined array, and filter categories with name 'Beverages'.

MongoDB

db.orders.aggregate([ { $lookup: { from: "[1]", localField: "categoryId", foreignField: "_id", as: "categoryInfo" } }, { $unwind: "[2]" }, { $match: { "categoryInfo.[3]": "Beverages" } } ])

Drag options to blanks, or click blank then click option'

Acategories

B$categoryInfo

Cname

Dproducts

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong collection name in 'from'.

Incorrect unwind path without '$'.

Filtering on wrong field name.

The from collection is 'categories'. The $unwind stage uses '$categoryInfo' to flatten the array. The $match filters by the 'name' field inside 'categoryInfo'.