MongoDB : Group by rounded value

Question

Let's take an example :

{ _id: "xx", total: 12345, name: "Bob" }
{ _id: "xa", total: 123, name: "Bob" }
{ _id: "xb", total: 1290, name: "Bob" }
{ _id: "xc", total: 1255, name: "Bob" }

I would like to have something like :

[
  { name: '100', count: 1 },
  { name: '1000', count: 2 },
  { name: '10000', count: 1 }
]

Actually i work with $project, $concat and $group. It works but i have 2 problem :

1. I would like to sort like my exemple (100, 1000, 10000) and i really don't know how i can do this
2. Actually it works with $concat. I have to write every case so it seems to be the wrong way. After some research i found that $round can take a negative number.

In my case : 123 could be 100 with $round(123, -2). But how can i have dynamically my negative factor ?

Yeah i really need help.

Thanks & Regards,

Answer 1

This aggregation should solve your issue:

db.collection.aggregate([
  {
    $addFields: {
      place: {
        $multiply: [
          {
            $add: [
              {
                $strLenCP: {
                  $toString: "$total",
                },
              },
              -1,
            ],
          },
          -1,
        ],
      },
    },
  },
  {
    $project: {
      name: {
        $round: ["$total", "$place"],
      },
    },
  },
  {
    $group: {
      _id: "$name",
      count: {
        $sum: 1,
      },
    },
  },
  {
    $project: {
      name: {
        $toString: "$_id",
      },
      count: 1,
      _id: 0,
    },
  },
  {
    $sort: {
      name: 1,
    },
  },
])