C100DEV Aggregation Framework Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  $unwind

  Why this is correct

  The $unwind stage deconstructs an array field from the input documents to output a document for each element. In this scenario, using $unwind on the `comments` array will produce one document per comment, preserving the parent post's fields such as `_id`. This exactly matches the requirement to list each comment as a separate document. Additional stages like $project can then reshape the output to include only the desired fields.

• ✗

  $project

  Why it's wrong here

  $project is used to reshape documents by including, excluding, or computing fields. It can include the `comments` array as a whole, but it does not create separate documents for each array element. To achieve one document per comment, you need a stage that expands arrays. $project alone cannot accomplish that; it would leave the array intact.

• ✗

  $replaceRoot

  Why it's wrong here

  $replaceRoot is used to replace the input document with a specified embedded document. It does not iterate over arrays. While it could promote a subdocument to the top level, it would only work if there is a single embedded document, not an array of comments. To expand an array, $unwind is required before or after $replaceRoot. Thus, $replaceRoot alone does not satisfy the requirement.

• ✗

  $group

  Why it's wrong here

  $group is used to aggregate documents by a specified key and perform accumulations, such as sum or average. It does not expand arrays into multiple documents. In this scenario, using $group would combine documents, not separate comments. Therefore, it cannot produce one document per comment. The requirement is to flatten an array, which $group does not do.