Equivalent of window function

Working with Data
1

I have following json structure:

image

I want to group it into this format (column VisitNo):
image
How can I do this using aggregation framework? Can’t find any similar example. I thought maybe to use the $accumulator but don’t know how to do it.

I want an equivalent of sql window function:

select u.RootId, u.VisitDate, count(u.VisitDate) over(partition by u.rootid order by u.visitdate) as VisitNo
From dbo.ProspectiveFollowUp u
2

I finally got it to work. But is there any othere, maybe better solution? Without the index usage?

db.dbo_ObservationJSON.aggregate([
  { $unwind: '$values' },
  { $unwind: { path: '$values.ProspectiveFollowUp', "includeArrayIndex": "index" } },
  {
    $project: {
      _id: 0,
      // Id: '$values.ProspectiveFollowUp.Id',
      RootId: '$values.ProspectiveFollowUp.RootId',
      VisitDate: '$values.ProspectiveFollowUp.VisitDate',
      VisitNo: { $add: ['$index', 1] }
    }
  },
  {
    $sort: {
      RootId: 1
    }
  }
])