$first and $last

$first and $last take a value from the start or end of each group. Neither has any idea what “first” means on its own - they read whatever order the documents arrived in, which makes the $sort before them part of the answer, not a formatting step.

Suppose:

{
  $sort: {
    createdAt: -1
  }
}

Then:

{
  $group: {
    _id: "$userId",

    latestOrder: {
      $first: "$_id"
    }
  }
}

Because we sorted newest first:

$first = newest order

If sorted ascending:

$first = oldest order

Important interview rule

$first and $last only make sense when you understand the order of documents entering the $group.

Try it

db.orders.aggregate([
  { $sort: { createdAt: -1 } },
  {
    $group: {
      _id: "$userId",
      latestOrder: { $first: "$_id" },
      latestAt: { $first: "$createdAt" }
    }
  },
  { $sort: { _id: 1 } },
  { $limit: 5 }
])

Now change the first $sort to createdAt: 1 and run it again. Not one character of the $group changed, and every answer did.