$lookup with pipeline (advanced join) in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a basic $lookup to join 'orders' with 'customers' on 'customerId'.

MongoDB

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

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

AcustomerId

BorderId

Cname

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using a field that does not exist in 'customers' like 'orderId'.

Mixing up localField and foreignField names.

The localField in 'orders' matches the foreignField in 'customers'. Both are 'customerId' to join correctly.

2fill in blank

medium

Complete the $lookup stage to use a pipeline that matches 'orders' with 'products' by 'productId'.

MongoDB

db.orders.aggregate([{ $lookup: { from: 'products', let: { pid: '$productId' }, pipeline: [ { $match: { $expr: { $eq: ['$_id', '[1]'] } } } ], as: 'productDetails' } }])

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

A$_id

B$productId

C$$pid

D$$productId

Attempts:

3 left

💡 Hint

Common Mistakes

Using single dollar sign like '$pid' instead of double '$$pid'.

Referencing the field directly without using the variable.

Inside the pipeline, $$pid refers to the variable defined in let which holds the 'productId' from 'orders'.

3fill in blank

hard

Fix the error in the $lookup pipeline to correctly filter 'reviews' for each 'product' by matching 'productId'.

MongoDB

db.products.aggregate([{ $lookup: { from: 'reviews', let: { prodId: '$_id' }, pipeline: [ { $match: { $expr: { $eq: ['$productId', '[1]'] } } } ], as: 'productReviews' } }])

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

A$prodId

B$$prodId

C$_id

D$$productId

Attempts:

3 left

💡 Hint

Common Mistakes

Using single dollar sign '$prodId' which is invalid.

Using the field name 'productId' directly instead of the variable.

Inside the pipeline, variables from let must be referenced with $$. So $$prodId is correct.

4fill in blank

hard

Fill both blanks to create a $lookup pipeline that matches 'orders' with 'items' where 'itemId' equals the order's 'itemId' and filters items with quantity greater than 5.

MongoDB

db.orders.aggregate([{ $lookup: { from: 'items', let: { oid: '$itemId' }, pipeline: [ { $match: { $expr: { $and: [ { $eq: ['[1]', '[2]'] }, { $gt: ['$quantity', 5] } ] } } } ], as: 'matchedItems' } }])

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

A$$oid

B$itemId

C$$itemId

D$_id

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of the fields in $eq.

Using single dollar sign for the variable.

The first blank is the field in 'items' collection, which is $_id (assuming item id is _id). The second blank is the variable from let, which is $$oid.

5fill in blank

hard

Fill all three blanks to create a $lookup pipeline that joins 'users' with 'orders' where 'userId' matches and only includes orders with status 'shipped'.

MongoDB

db.users.aggregate([{ $lookup: { from: 'orders', let: { uid: '$_id' }, pipeline: [ { $match: { $expr: { $eq: ['[1]', '[2]'] } } }, { $match: { status: [3] } } ], as: 'shippedOrders' } }])

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

A$userId

B$$uid

C'shipped'

D'delivered'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name in $eq.

Forgetting quotes around the status string.

Using single dollar sign for the variable.

The $eq compares the 'orders.userId' field with the variable $$uid. The status filter matches the string 'shipped'.