C100DEV Aggregation Framework Practice Question
Exhibit
db.collection.aggregate([ { $group: { _id: "$category", total: { $sum: "$price" } } } ])Refer to the exhibit. What happens to documents in the collection that do not contain the 'category' field?
⚠ Common exam trap
Candidates often assume documents missing the grouping field are excluded from the results, not realizing that MongoDB treats missing fields as null and groups them together under the null key.
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
✓
They are assigned to a group with _id: null.
When grouping by a field, MongoDB treats missing fields or null values as having the value 'null'. Therefore, all documents lacking a 'category' field will be grouped together under the ID 'null'. This behavior is consistent and ensures no documents are silently dropped during a $group operation, allowing developers to identify and handle data quality issues where mandatory fields might be missing from certain records.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
They are excluded from the results.
Why it's wrong here
MongoDB does not silently drop documents during a $group stage. If the grouping field is missing, the engine assigns them to a group identified by 'null'. This ensures data integrity and prevents the accidental loss of records that simply lack specific field values in their current schema structure.
- ✓
They are assigned to a group with _id: null.
Why this is correct
When the field specified in the _id expression of a $group stage is missing, MongoDB treats the value as null. All such documents are subsequently aggregated into a single group where the _id field is set to null, effectively capturing all documents that lack the 'category' key.
- ✗
The aggregation throws an error.
Why it's wrong here
The aggregation engine does not require fields to be present in every document. It gracefully handles missing fields by substituting them with a null value for the purpose of the group identification. Consequently, the query runs successfully without error, aggregating the incomplete documents into a null group.
- ✗
They are grouped individually as unique documents.
Why it's wrong here
Grouping logic aggregates documents based on the equality of the _id expression. Because all missing fields evaluate to null, they are considered equal in the context of the grouping operation. Therefore, they are merged into a single bucket rather than being treated as separate individual entries.
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.