$unwind for flattening arrays in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to unwind the 'items' array in the aggregation pipeline.

MongoDB

db.orders.aggregate([{ $[1]: "$items" }])

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

Amatch

Bunwind

Cgroup

Dproject

Attempts:

3 left

💡 Hint

Common Mistakes

Using $match instead of $unwind will filter documents but not flatten arrays.

Using $group or $project won't flatten arrays directly.

The $unwind stage is used to flatten an array field in documents, creating a separate document for each element.

2fill in blank

medium

Complete the code to unwind the 'tags' array and preserve documents with empty or missing 'tags'.

MongoDB

db.products.aggregate([{ $unwind: { path: "$tags", [1]: true } }])

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

AallowDiskUse

BsortByCount

CpreserveNullAndEmptyArrays

DincludeArrayIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using includeArrayIndex adds the index but does not preserve empty arrays.

sortByCount and allowDiskUse are unrelated options.

The preserveNullAndEmptyArrays: true option keeps documents that have empty or missing arrays instead of removing them.

3fill in blank

hard

Fix the error in the unwind stage to correctly specify the array field path.

MongoDB

db.sales.aggregate([{ $unwind: "[1]" }])

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

A$items

Bitems

C"items"

Ditems[]

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the $ sign causes the stage to fail.

Using brackets or quotes incorrectly changes the meaning.

The $unwind stage requires the array field path as a string starting with a dollar sign, like "$items".

4fill in blank

hard

Fill both blanks to unwind the 'comments' array and include the array index as 'commentIndex'.

MongoDB

db.posts.aggregate([{ $unwind: { path: "$comments", [1]: "[2]" } }])

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

AincludeArrayIndex

BpreserveNullAndEmptyArrays

CcommentIndex

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing preserveNullAndEmptyArrays with includeArrayIndex.

Using a wrong field name for the index.

The includeArrayIndex option adds the array element's index to each output document, and you can name this field, here as commentIndex.

5fill in blank

hard

Fill all three blanks to unwind the 'reviews' array, preserve empty arrays, and include the index as 'reviewPos'.

MongoDB

db.books.aggregate([{ $unwind: { path: "$reviews", [1]: true, [2]: "[3]" } }])

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

ApreserveNullAndEmptyArrays

BincludeArrayIndex

CreviewPos

Dpath

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up option names or forgetting to quote the index field name.

Not preserving empty arrays when needed.

This unwind stage flattens the 'reviews' array, keeps documents with empty or missing arrays, and adds the index of each review as 'reviewPos'.