$replaceRoot for restructuring in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

$replaceRoot replaces the current document with a specified embedded document. It helps restructure documents by promoting a nested document to the top level.

beginner

How do you specify the new root document in $replaceRoot?

You use the newRoot field inside $replaceRoot to specify the embedded document that will become the new root.

beginner

Example: If a document is { _id: 1, info: { name: 'Alice', age: 30 } }, what will { $replaceRoot: { newRoot: '$info' } } produce?

The output document will be { name: 'Alice', age: 30 }. The info subdocument becomes the whole document.

intermediate

Can $replaceRoot be used to flatten nested documents?

Yes, by replacing the root with a nested document, you effectively flatten the structure by removing the outer layers.

intermediate

What happens if the newRoot field in $replaceRoot does not exist in the document?

The aggregation will return null for that document, effectively removing it from the output if followed by a $match or similar stage.

What is the main purpose of $replaceRoot in MongoDB?

A To delete documents from the collection

B To replace the current document with a nested document

C To add new fields to the document

D To sort documents by a field

$replaceRoot changes the root document to a specified embedded document, restructuring the output.

Which field do you use inside $replaceRoot to specify the new root document?

A rootDoc

B document

C replaceWith

D newRoot

The newRoot field defines the embedded document to become the new root.

Given { _id: 2, details: { city: 'NY', zip: 10001 } }, what does { $replaceRoot: { newRoot: '$details' } } output?

A { city: 'NY', zip: 10001 }

B { _id: 2 }

C { _id: 2, details: { city: 'NY', zip: 10001 } }

D null

The details subdocument becomes the whole document after $replaceRoot.

If the specified newRoot path does not exist in a document, what is the result?

A The original document is kept

B An error is thrown

C The document becomes <code>null</code>

D The document is duplicated

The document becomes null because the path does not exist.

Can $replaceRoot be used to promote a nested document to the top level?

A Yes

B No

C Only with <code>$project</code>

D Only with <code>$group</code>

$replaceRoot is designed to promote nested documents to the root level.

Explain how $replaceRoot works in MongoDB aggregation and give a simple example.

Think about changing the shape of documents by picking a nested part as the whole document.

What happens if the path specified in newRoot does not exist in a document during $replaceRoot?

Consider how missing data affects the output document.