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.

• ✓

  $sort by timestamp descending, then $group with { _id: "$sensorId", latestValue: { $first: "$value" }, avgValue: { $avg: "$value" } }

  Why this is correct

  Sorting by timestamp descending ensures that within each sensor group, the first document encountered by $first is the most recent reading. The $group stage then uses $first to capture that value and $avg to compute the average. This pipeline correctly produces the desired output for each sensor.

• ✗

  $group with { _id: "$sensorId", latestValue: { $max: "$value" }, avgValue: { $avg: "$value" } }

  Why it's wrong here

  $max returns the highest value, not the value associated with the most recent timestamp. The most recent reading could have a lower value, so $max does not satisfy the requirement. The average is correct, but the latest value is wrong. Therefore, this pipeline is incorrect.

• ✗

  $group with { _id: "$sensorId", latestValue: { $last: "$value" }, avgValue: { $avg: "$value" } }

  Why it's wrong here

  Without sorting, $last does not guarantee the most recent reading; it simply returns the last document in the group as encountered by the pipeline, which is not necessarily the latest by timestamp. The average would be correct, but the latest value would be unreliable. Thus, this pipeline does not meet the requirement.

• ✗

  $sort by timestamp ascending, then $group with { _id: "$sensorId", latestValue: { $last: "$value" }, avgValue: { $avg: "$value" } }

  Why it's wrong here

  Sorting ascending and using $last would indeed capture the most recent reading, but the average is computed correctly. However, this approach relies on the sort order and $last, which works but is less explicit than using $first after a descending sort. Both are valid, but only one option is correct; here the descending sort with $first is more conventional and clear.