The accumulator operators

An accumulator is what you compute for each group. MongoDB ships a long list of them; this lesson covers the working set rather than the reference manual.

These are the ones worth knowing cold:

$sum
$avg
$min
$max
$push
$addToSet
$first
$last

Example:

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

    total: {
      $sum: "$amount"
    },

    average: {
      $avg: "$amount"
    },

    biggestOrder: {
      $max: "$amount"
    },

    smallestOrder: {
      $min: "$amount"
    }
  }
}

Try it

Four of them at once, per user:

db.orders.aggregate([
  { $unwind: "$items" },
  {
    $group: {
      _id: "$userId",
      total: {
        $sum: {
          $multiply: ["$items.price", "$items.quantity"]
        }
      },
      average: { $avg: "$items.price" },
      biggest: { $max: "$items.price" },
      smallest: { $min: "$items.price" }
    }
  },
  { $sort: { total: -1 } },
  { $limit: 5 }
])

One pass over the data produces all four. They are not four queries.