C100DEV Aggregation Framework Practice Question
Which aggregation stage allows you to replace the root of the document with a specified document or field?
⚠ Common exam trap
Candidates often mix up $replaceRoot with $project or $addFields, thinking those stages can completely swap the document root structure.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
$replaceRoot
The $replaceRoot stage is used to restructure a document by promoting a nested field to the top level. This is extremely useful for flattening complex, deeply nested JSON structures retrieved from other collections or generated through aggregations. By simplifying the document schema, it makes subsequent pipeline stages cleaner and easier to manage, which is a best practice for maintaining readable and maintainable aggregation pipelines in complex applications.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
$project
Why it's wrong here
$project is for including or excluding specific fields, not for replacing the document root. While $project can reshape a document, $replaceRoot is specifically designed for structural promotion, making it the more precise and intended operator for this type of document restructuring task.
- ✓
$replaceRoot
Why this is correct
$replaceRoot effectively promotes a nested document to the top level of the aggregation stream. This is the correct choice for restructuring documents by removing the original parent structure and placing a target inner document at the top level for further processing and analysis.
- ✗
$set
Why it's wrong here
$set is a shorthand for $addFields, used to add or update fields in a document. It does not possess the capability to discard the entire structure and replace the document root, making it inappropriate for restructuring the top-level schema of the result documents.
- ✗
$unwind
Why it's wrong here
$unwind is for splitting arrays into individual documents. It has no functional overlap with replacing the root of a document. Using $unwind for root replacement would result in unexpected multiplication of documents rather than the desired structural change to the individual document's root field.
About these practice questions
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DEV practice question is part of Courseiva's free MongoDB certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the C100DEV exam.