$unwind for flattening arrays in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The $unwind stage takes an array field from each document and creates a separate document for each element in that array. This 'flattens' the array so you can work with each element individually.

intermediate

How does $unwind affect documents that have an empty array or missing array field?

By default, $unwind removes documents where the array is empty or the field is missing. You can change this behavior with the preserveNullAndEmptyArrays option to keep those documents.

beginner

Example: Given a document { name: 'Alice', hobbies: ['reading', 'swimming'] }, what will $unwind: '$hobbies' produce?

It will produce two documents:
{ name: 'Alice', hobbies: 'reading' }
{ name: 'Alice', hobbies: 'swimming' }

intermediate

What option can you use with $unwind to keep documents that have empty or missing arrays?

You can use { preserveNullAndEmptyArrays: true } inside $unwind to keep documents even if the array is empty or missing.

beginner

Why is $unwind useful when working with arrays in MongoDB?

It helps to break down arrays into individual elements so you can filter, group, or sort based on each element separately, making data analysis easier.

What does the $unwind stage do in MongoDB?

A Joins two collections together

B Sorts documents by array length

C Deletes documents with empty arrays

D Splits array elements into separate documents

$unwind splits each element of an array into its own document, effectively flattening the array.

If a document has an empty array field, what happens by default when using $unwind on that field?

A The document is removed from the output

B The document is kept unchanged

C The array is filled with nulls

D An error is thrown

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

Which option allows $unwind to keep documents with empty or missing arrays?

A keepEmptyArrays: true

B includeEmpty: true

C preserveNullAndEmptyArrays: true

D allowEmpty: true

The correct option is preserveNullAndEmptyArrays: true.

Given { name: 'Bob', tags: ['fun', 'smart'] }, what does $unwind: '$tags' produce?

A One document with tags as an array

B Two documents, each with one tag

C No documents

D One document with tags as a string 'fun, smart'

$unwind creates one document per array element.

Why would you use $unwind in an aggregation pipeline?

A To flatten arrays for easier filtering and grouping

B To delete array fields

C To combine multiple arrays into one

D To rename fields

$unwind helps to flatten arrays so you can work with each element individually.

Explain how the $unwind stage works in MongoDB and why it is useful.

Think about how arrays become easier to work with when broken down.

Describe what happens to documents with empty or missing arrays when using $unwind and how to change this behavior.

Consider the default behavior and the option to keep those documents.

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