$replaceRoot for restructuring in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this $replaceRoot aggregation?

Given the collection documents:

{ "_id": 1, "user": { "name": "Alice", "age": 30 }, "score": 85 }

What will be the output of this aggregation?

[{ $replaceRoot: { newRoot: "$user" } }]

MongoDB

[{ $replaceRoot: { newRoot: "$user" } }]

A [{ "name": "Alice", "age": 30 }]

B [{ "user": { "name": "Alice", "age": 30 }, "score": 85 }]

C [{ "_id": 1, "name": "Alice", "age": 30 }]

D [{ "name": "Alice", "age": 30, "score": 85 }]

💡 Hint

Think about what $replaceRoot does with the specified newRoot field.

$replaceRoot replaces the entire document with the specified field's content. Here, it replaces the root with the 'user' subdocument.

❓ query_result

intermediate

What does this $replaceRoot pipeline produce?

Given documents:

{ "_id": 2, "order": { "id": 101, "items": 3 }, "status": "shipped" }

Run this pipeline:

[{ $replaceRoot: { newRoot: { $mergeObjects: ["$order", { "status": "$status" }] } } }]

MongoDB

[{ $replaceRoot: { newRoot: { $mergeObjects: ["$order", { "status": "$status" }] } } }]

A [{ "id": 101, "items": 3, "status": "shipped" }]

B [{ "order": { "id": 101, "items": 3 }, "status": "shipped" }]

C [{ "id": 101, "items": 3 }]

D [{ "status": "shipped" }]

💡 Hint

Look at how $mergeObjects combines fields from two documents.

$mergeObjects merges the 'order' subdocument with a new document containing 'status'. $replaceRoot then sets this merged document as the root.

📝 Syntax

advanced

Which $replaceRoot usage is syntactically correct?

Choose the option with correct syntax for $replaceRoot stage in MongoDB aggregation.

A { $replaceRoot: "$details" }

B { $replaceRoot: { root: "$details" } }

C { $replaceRoot: { newRoot: details } }

D { $replaceRoot: { newRoot: "$details" } }

💡 Hint

Check the exact field name and value type required by $replaceRoot.

$replaceRoot requires an object with a 'newRoot' field whose value is an expression (usually a string path).

❓ optimization

advanced

How to optimize restructuring with $replaceRoot and $project?

You want to restructure documents by replacing the root with the 'profile' field but also keep the original '_id'. Which pipeline is best?

A [{ $replaceRoot: { newRoot: "$profile" } }, { $project: { _id: 1 } }]

B [{ $replaceRoot: { newRoot: { $mergeObjects: ["$profile", { _id: "$_id" }] } } }]

C [{ $replaceRoot: { newRoot: "$profile" } }, { $addFields: { _id: "$_id" } }]

D [{ $project: { _id: 1, profile: 1 } }, { $replaceRoot: { newRoot: "$profile" } }]

💡 Hint

Consider how to keep _id while replacing root in one step.

Option B merges 'profile' with an object containing '_id', then replaces root with the merged object, preserving _id efficiently.

🧠 Conceptual

expert

What error occurs if $replaceRoot newRoot field is missing?

Consider this aggregation stage:

{ $replaceRoot: { } }

What error will MongoDB raise when running this?

A TypeError: Cannot read property 'newRoot' of undefined

B SyntaxError: Unexpected end of stage specification

C MongoError: 'newRoot' is required for $replaceRoot stage

D No error, pipeline runs but documents are unchanged

💡 Hint

Think about required fields for $replaceRoot stage.

The $replaceRoot stage requires the 'newRoot' field. Omitting it causes a MongoError indicating the missing required field.