$unwind for flattening arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this $unwind operation?

Given the collection documents:

{ _id: 1, fruits: ["apple", "banana"] }

What will be the result of this aggregation?

[{ $unwind: "$fruits" }]

A [{ _id: 1, fruits: "apple" }, { _id: 1, fruits: "banana" }]

B [{ _id: 1, fruits: ["apple"] }, { _id: 1, fruits: ["banana"] }]

C [{ _id: 1, fruits: "apple" }]

D [{ _id: 1, fruits: ["apple", "banana"] }]

💡 Hint

Think about how $unwind breaks down arrays into separate documents.

The $unwind stage creates one document per element in the array, replacing the array field with the element value.

📝 Syntax

intermediate

Which $unwind syntax is correct to flatten the 'tags' array?

Choose the correct $unwind stage to flatten the 'tags' array in documents.

A { $unwind: ["$tags"] }

B { $unwind: "$tags" }

C { $unwind: tags }

D { $unwind: { path: "tags" } }

💡 Hint

Remember $unwind expects a string path with $ prefix or an object with path field.

The correct syntax uses a string with $ prefix to specify the array field to unwind.

❓ query_result

advanced

What is the output when using $unwind with preserveNullAndEmptyArrays?

Given documents:

{ _id: 1, items: ["pen", "pencil"] }, { _id: 2 }

What is the result of:

[{ $unwind: { path: "$items", preserveNullAndEmptyArrays: true } }]

A [{ _id: 1, items: ["pen", "pencil"] }, { _id: 2 }]

B [{ _id: 1, items: "pen" }, { _id: 1, items: "pencil" }]

C [{ _id: 1, items: "pen" }, { _id: 1, items: "pencil" }, { _id: 2, items: null }]

D [{ _id: 1, items: "pen" }, { _id: 2, items: null }]

💡 Hint

PreserveNullAndEmptyArrays keeps documents without the array or with empty arrays.

Documents without the array field are included with the field set to null when preserveNullAndEmptyArrays is true.

❓ optimization

advanced

How to optimize $unwind when array field might be missing or empty?

You want to unwind the 'comments' array but keep documents without comments or with empty arrays. Which $unwind stage is best?

A { $unwind: { path: "$comments" } }

B { $unwind: "$comments" }

C { $unwind: { path: "$comments", preserveNullAndEmptyArrays: false } }

D { $unwind: { path: "$comments", preserveNullAndEmptyArrays: true } }

💡 Hint

Consider documents that may not have the array field or have it empty.

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

🧠 Conceptual

expert

What happens if you $unwind a non-array field?

If a document has a field 'score' with a single integer value (not an array), what happens when you apply:

[{ $unwind: "$score" }]

A The document is removed from the output because 'score' is not an array.

B The aggregation throws an error because $unwind expects an array field.

C The document is duplicated once for the single value of 'score'.

D The document is passed through unchanged because 'score' is not an array.

💡 Hint

Think about how $unwind treats fields that are not arrays.

If the field is not an array, $unwind removes the document from the output unless preserveNullAndEmptyArrays is true.

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