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.

• ✓

  { $dateTrunc: { date: "$recordedAt", unit: "minute", binSize: 15 } }

  Why this is correct

  The $dateTrunc operator truncates a date to the specified unit and binSize, producing a date rounded down to the nearest 15-minute boundary. This creates a consistent grouping key for intervals like 10:00, 10:15, 10:30. It is the correct tool for time-bucketing in aggregation pipelines.

• ✗

  { $dateFromParts: { year: { $year: "$recordedAt" }, month: { $month: "$recordedAt" }, day: { $dayOfMonth: "$recordedAt" }, hour: { $hour: "$recordedAt" }, minute: { $multiply: [ { $floor: { $divide: [ { $minute: "$recordedAt" }, 15 ] } }, 15 ] } } }

  Why it's wrong here

  This complex expression manually reconstructs a date with minutes rounded down to the nearest multiple of 15. While it could work, it is overly verbose and error-prone, especially with edge cases like hour rollover. The $dateTrunc operator is the intended, simpler solution.

• ✗

  { $subtract: [ "$recordedAt", { $mod: [ { $minute: "$recordedAt" }, 15 ] } ] }

  Why it's wrong here

  This expression attempts to subtract the remainder of minutes modulo 15, but it operates on the minute component alone, not the full date. It would not correctly adjust the timestamp to the 15-minute boundary and would likely produce incorrect dates, especially across hour boundaries.

• ✗

  { $dateToString: { format: "%Y-%m-%d %H:%M", date: "$recordedAt" } }

  Why it's wrong here

  While $dateToString can format dates, it does not round to 15-minute intervals. It would produce a string with the exact minute of each reading, resulting in many distinct groups rather than 15-minute buckets. You would need additional string manipulation to achieve the desired grouping.