$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.