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.

• ✓

  { $sum: 1 } and { $max: "$timestamp" }

  Why this is correct

  Using $sum with a literal 1 counts one per document, yielding the total number of readings in each group. Applying $max to the timestamp field returns the largest (most recent) timestamp value per group, which is exactly the latest reading time. Both accumulators are valid within $group and produce the two required computed fields in a single pass.

• ✗

  { $count: 1 } and { $last: "$timestamp" }

  Why it's wrong here

  $count is an accumulator introduced in MongoDB 5.0, but using it as { $count: 1 } is not a valid expression syntax for counting within $group; the correct form is { $count: {} }. Even if valid, $last returns the timestamp of the last document encountered, which is not guaranteed to be the most recent unless the input is sorted first.

• ✗

  { $sum: "$timestamp" } and { $max: "$timestamp" }

  Why it's wrong here

  Applying $sum to the timestamp field would attempt to add numeric timestamp values, producing a meaningless sum rather than a count of readings. The $max portion is correct for the latest reading, but the counting requirement fails because $sum on a timestamp field does not count documents. A literal 1 is needed to count each document.

• ✗

  { $size: 1 } and { $max: "$timestamp" }

  Why it's wrong here

  $size is not an accumulator and cannot be used inside $group; it is an array expression operator that returns the number of elements in an array. Using it here would raise an error. Even conceptually, there is no way to count group members with $size. The $max portion is valid, but the pair as a whole is invalid.