$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.

Practice

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

easy

A. It groups documents by a specified key.

B. It filters documents based on a condition.

C. It replaces the entire document with the specified embedded document.

D. It sorts documents by a specified field.

Solution

1. Step 1: Understand the purpose of $replaceRoot

   $replaceRoot replaces the whole document with a specified nested document inside it.

2. Step 2: Compare with other stages

   Filtering, sorting, and grouping are done by $match, $sort, and $group respectively, not $replaceRoot.

3. Final Answer:

   It replaces the entire document with the specified embedded document. -> Option C

4. Quick Check:

   $replaceRoot changes document root [OK]

Hint: Remember: $replaceRoot swaps the whole document root [OK]

Common Mistakes:

• Confusing $replaceRoot with filtering or sorting stages
• Thinking it only modifies part of the document
• Assuming it adds fields instead of replacing

2. Which of the following is the correct syntax to use $replaceRoot to replace the root with the details field?

easy

A. { $replaceRoot: { newRoot: "$details" } }

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

C. { $replaceRoot: { newRoot: details } }

D. { $replaceRoot: { newRoot: "$details" }, $match: {} }

Solution

1. Step 1: Recall correct $replaceRoot syntax

   The correct syntax requires newRoot with a string path starting with "$" to specify the field.

2. Step 2: Analyze options

   { $replaceRoot: { newRoot: "$details" } } uses newRoot: "$details" which is correct. { $replaceRoot: { root: "$details" } } uses wrong key root. { $replaceRoot: { newRoot: details } } misses quotes around field path. { $replaceRoot: { newRoot: "$details" }, $match: {} } adds an unrelated $match inside the same stage, which is invalid.

3. Final Answer:

   { $replaceRoot: { newRoot: "$details" } } -> Option A

4. Quick Check:

   Use newRoot: "$field" syntax [OK]

Hint: Use newRoot: "$fieldName" exactly [OK]

Common Mistakes:

• Omitting quotes around field path
• Using wrong key like 'root' instead of 'newRoot'
• Combining stages incorrectly

3. Given the collection documents:

{ "name": "Alice", "info": { "age": 30, "city": "NY" } }

What will be the output of this aggregation?

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

medium

A. [{ "name": "Alice", "info": { "age": 30, "city": "NY" } }]

B. [{ "age": 30, "city": "NY" }]

C. [{ "info": { "age": 30, "city": "NY" } }]

D. [{ "age": 30 }]

Solution

1. Step 1: Understand $replaceRoot effect

   The stage replaces the whole document with the content of the info field.

2. Step 2: Apply to given document

   Original document has info as { "age": 30, "city": "NY" }, so output document is exactly that object.

3. Final Answer:

   [{ "age": 30, "city": "NY" }] -> Option B

4. Quick Check:

   Root replaced by info field content [OK]

Hint: Output matches the nested field content exactly [OK]

Common Mistakes:

• Expecting original fields to remain
• Confusing nested field with root
• Selecting wrong output format

4. You wrote this aggregation but it throws an error:

[{ $replaceRoot: { newRoot: info } }]

What is the error and how to fix it?

medium

A. Missing quotes around field path; fix with newRoot: "$info".

B. Wrong operator; use $setRoot instead of $replaceRoot.

C. Field info does not exist; add it first.

D. Aggregation pipeline must be an object, not an array.

Solution

1. Step 1: Identify syntax error in newRoot value

   The field path must be a string starting with "$" to indicate a field reference.

2. Step 2: Correct the syntax

   Change newRoot: info to newRoot: "$info" to fix the error.

3. Final Answer:

   Missing quotes around field path; fix with newRoot: "$info". -> Option A

4. Quick Check:

   Field paths need quotes and $ prefix [OK]

Hint: Always quote field paths with $ prefix [OK]

Common Mistakes:

• Omitting quotes around field names
• Confusing $replaceRoot with $setRoot
• Misunderstanding pipeline array structure

5. You have documents:

{ "user": { "id": 1, "name": "Bob" }, "score": 50 }

How do you write an aggregation pipeline using $replaceRoot to make the output documents only contain the user fields plus the score field at the root level?

hard

A. [{ $replaceRoot: { newRoot: "$score" } }]

B. [{ $replaceRoot: { newRoot: "$user" } }, { $addFields: { score: "$score" } }]

C. [{ $project: { user: 1, score: 1 } }]

D. [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }]

Solution

1. Step 1: Understand the goal

   We want to flatten user fields to root and keep score alongside.

2. Step 2: Use $mergeObjects inside $replaceRoot

   $mergeObjects combines user object and { score: "$score" } into one root document.

3. Step 3: Check options

   [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }] correctly merges and replaces root. [{ $replaceRoot: { newRoot: "$user" } }, { $addFields: { score: "$score" } }] tries to add fields after replacing root, but score is lost after first stage. [{ $project: { user: 1, score: 1 } }] only projects fields, not restructuring. [{ $replaceRoot: { newRoot: "$score" } }] replaces root with score only, losing user.

4. Final Answer:

   [{ $replaceRoot: { newRoot: { $mergeObjects: ["$user", { score: "$score" }] } } }] -> Option D

5. Quick Check:

   Use $mergeObjects to combine fields [OK]

Hint: Combine objects with $mergeObjects inside $replaceRoot [OK]

Common Mistakes:

• Replacing root with only one field losing others
• Trying to add fields after $replaceRoot without preserving data
• Using $project instead of restructuring