Counting and summing

Counting and summing are the two things you will do with $group more than everything else combined. They are the same operator: $sum: 1 counts, $sum: "$field" totals.

Now let’s count orders per user.

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

    orderCount: {
      $sum: 1
    }
  }
}

Result:

[
  {
    _id: 101,
    orderCount: 2
  },
  {
    _id: 102,
    orderCount: 1
  }
]

Mental translation:

Group by userId

For every document:
  add 1

This is very similar to:

GROUP BY userId
COUNT(*)

Sum a field

Suppose documents:

{
  userId: 101,
  amount: 500
}

{
  userId: 101,
  amount: 300
}

Pipeline:

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

    totalSpent: {
      $sum: "$amount"
    }
  }
}

Result:

{
  _id: 101,
  totalSpent: 800
}

Try it

db.orders.aggregate([
  {
    $group: {
      _id: "$userId",
      orderCount: { $sum: 1 }
    }
  },
  { $sort: { orderCount: -1 } },
  { $limit: 5 }
])

The busiest five users. Change $sum: 1 to $sum: "$rating" and the same pipeline totals a field instead of counting documents.