C100DEV Aggregation Framework Practice Question
A pipeline on a `sales` collection must return only the `item` and `price` fields for all documents where `price` is greater than 50, and it should not return the `_id` field. Which pipeline achieves this?
⚠ Common exam trap
The trap here is forgetting that `_id` is included by default in a `$project` inclusion projection, so it must be explicitly excluded when not wanted.
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
✓
[ { $match: { price: { $gt: 50 } } }, { $project: { _id: 0, item: 1, price: 1 } } ]
The pipeline should filter with `$match` on `price` first, then use `$project` to include `item` and `price` while excluding `_id`. This returns only the requested fields for documents meeting the condition. Projecting before matching or using `$unset` does not satisfy the requirement to return only those two fields without `_id`.
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: { _id: 0, item: 1, price: 1 } }, { $match: { price: { $gt: 50 } } } ]
Why it's wrong here
This pipeline projects first, then filters. While it may still return the correct fields, it is less efficient because `$match` after `$project` may not use the index on `price` as effectively. The aggregation optimizer can sometimes reorder these, but explicit `$match` first is preferred for performance and clarity.
- ✗
[ { $match: { price: { $gt: 50 } } }, { $project: { item: 1, price: 1 } } ]
Why it's wrong here
This pipeline filters correctly but does not exclude `_id`. In MongoDB, `_id` is included by default in projection unless explicitly set to 0. The scenario requires that `_id` not be returned, so this pipeline would include an unwanted field and fail the requirement.
- ✓
[ { $match: { price: { $gt: 50 } } }, { $project: { _id: 0, item: 1, price: 1 } } ]
Why this is correct
The `$match` stage filters documents where `price` is greater than 50, and the `$project` stage includes only `item` and `price` while explicitly excluding `_id` with `_id: 0`. This returns exactly the required fields for qualifying documents. The order is efficient because filtering happens before projection.
- ✗
[ { $match: { price: { $gt: 50 } } }, { $unset: ["_id", "department"] } ]
Why it's wrong here
The `$unset` stage removes fields but does not include only `item` and `price`. It would leave all other fields except `_id` and `department`, so the output would contain extra fields not requested. The scenario asks for only `item` and `price`, so `$project` with inclusion is the correct approach.
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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
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