$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'.

Practice

1. What does the $unwind stage do in a MongoDB aggregation pipeline?

easy

A. It turns each element of an array field into a separate document.

B. It groups documents by a specified field.

C. It sorts documents in ascending order.

D. It removes duplicate documents from the collection.

Solution

1. Step 1: Understand the purpose of $unwind

   $unwind takes an array field and creates a new document for each element in that array.

2. Step 2: Compare with other pipeline stages

   Grouping, sorting, and removing duplicates are done by other stages like $group, $sort, and $match, not $unwind.

3. Final Answer:

   It turns each element of an array field into a separate document. -> Option A

4. Quick Check:

   $unwind flattens arrays = B [OK]

Hint: Remember: $unwind splits arrays into separate documents [OK]

Common Mistakes:

• Confusing $unwind with $group or $sort
• Thinking $unwind removes duplicates
• Assuming $unwind sorts documents

2. Which of the following is the correct syntax to use $unwind on the field items in an aggregation pipeline?

easy

A. { $unwind: "$items" }

B. { $unwind: { path: "$items" } }

C. { $unwind: items }

D. { $unwind: { path: "items" } }

Solution

1. Step 1: Recall correct $unwind syntax

   The $unwind stage requires the field path as a string with a $ prefix inside an object with key path.

2. Step 2: Evaluate each option

   { $unwind: "$items" } misses the path key. { $unwind: items } lacks quotes and $ prefix. { $unwind: { path: "items" } } misses the $ prefix. Only { $unwind: { path: "$items" } } correctly uses { path: "$items" }.

3. Final Answer:

   { $unwind: { path: "$items" } } -> Option B

4. Quick Check:

   Use { path: "$field" } for $unwind [OK]

Hint: Use { path: "$field" } syntax for $unwind [OK]

Common Mistakes:

• Omitting quotes or $ in field path
• Using string without path key
• Passing field name without $ prefix

3. Given the collection documents:

{ "_id": 1, "tags": ["red", "blue"] }

{ "_id": 2, "tags": ["green"] }

What will be the output documents after applying this pipeline?

[ { $unwind: "$tags" } ]

medium

A. [{ "_id": 1, "tags": "blue" }, { "_id": 2, "tags": "green" }]

B. [{ "_id": 1, "tags": ["red", "blue"] }, { "_id": 2, "tags": ["green"] }]

C. [{ "_id": 1, "tags": "red" }, { "_id": 2, "tags": "green" }]

D. [{ "_id": 1, "tags": "red" }, { "_id": 1, "tags": "blue" }, { "_id": 2, "tags": "green" }]

Solution

1. Step 1: Understand $unwind effect on arrays

   $unwind creates one document per array element, duplicating other fields.

2. Step 2: Apply $unwind to each document

   Document 1 with tags ["red", "blue"] becomes two documents with tags "red" and "blue". Document 2 with tags ["green"] becomes one document with tag "green".

3. Final Answer:

   [{ "_id": 1, "tags": "red" }, { "_id": 1, "tags": "blue" }, { "_id": 2, "tags": "green" }] -> Option D

4. Quick Check:

   Each array element becomes a document = A [OK]

Hint: Each array item becomes a separate document after $unwind [OK]

Common Mistakes:

• Expecting original array to remain intact
• Missing duplicated documents for multiple array items
• Confusing $unwind with $group

4. You run this aggregation pipeline:

[ { $unwind: "$comments" } ]

But some documents have no comments field or have it as an empty array. What problem occurs and how to fix it?

medium

A. Documents without comments are dropped; fix by adding preserveNullAndEmptyArrays: true in $unwind.

B. Syntax error occurs; fix by quoting the field name.

C. Documents are duplicated; fix by removing $unwind.

D. No problem occurs; $unwind handles missing fields automatically.

Solution

1. Step 1: Identify behavior of $unwind on missing or empty arrays

   By default, $unwind removes documents where the array field is missing or empty.

2. Step 2: Use preserveNullAndEmptyArrays option to keep those documents

   Adding preserveNullAndEmptyArrays: true keeps documents without the array or with empty arrays in the output.

3. Final Answer:

   Documents without comments are dropped; fix by adding preserveNullAndEmptyArrays: true in $unwind. -> Option A

4. Quick Check:

   Use preserveNullAndEmptyArrays to keep empty/missing arrays [OK]

Hint: Add preserveNullAndEmptyArrays:true to keep missing arrays [OK]

Common Mistakes:

• Ignoring dropped documents without array
• Assuming $unwind never drops documents
• Confusing syntax errors with missing field issues

5. You have documents with a field orders which is an array of objects:

{ "_id": 1, "orders": [{ "item": "apple", "qty": 5 }, { "item": "banana", "qty": 3 }] }

You want to create a list of all items ordered with their quantities as separate documents, but also keep documents that have no orders. Which aggregation pipeline correctly does this?

hard

A. [ { $unwind: { path: "$orders" } }, { $match: { orders: { $exists: true } } } ]

B. [ { $unwind: "$orders" }, { $project: { item: "$orders.item", qty: "$orders.qty" } } ]

C. [ { $unwind: { path: "$orders", preserveNullAndEmptyArrays: true } }, { $project: { item: "$orders.item", qty: "$orders.qty" } } ]

D. [ { $project: { item: "$orders.item", qty: "$orders.qty" } } ]

Solution

1. Step 1: Use $unwind with preserveNullAndEmptyArrays to keep documents without orders

   [ { $unwind: { path: "$orders", preserveNullAndEmptyArrays: true } }, { $project: { item: "$orders.item", qty: "$orders.qty" } } ] uses preserveNullAndEmptyArrays: true to keep documents that have no orders.

2. Step 2: Project only the needed fields from the unwound orders array

   The $project stage extracts item and qty from each order object.

3. Final Answer:

   [ { $unwind: { path: "$orders", preserveNullAndEmptyArrays: true } }, { $project: { item: "$orders.item", qty: "$orders.qty" } } ] -> Option C

4. Quick Check:

   Unwind with preserveNullAndEmptyArrays + project fields = D [OK]

Hint: Use preserveNullAndEmptyArrays:true to keep docs without array [OK]

Common Mistakes:

• Omitting preserveNullAndEmptyArrays and losing documents
• Projecting fields before unwind causing errors
• Using $match to filter instead of preserveNullAndEmptyArrays