Mongodb Aggregate events between two events

Question

I have a collection made like this

[
  {timestamp: xxx, type:'start'},
  {timestamp: xxx, type:'log'},
  {timestamp: xxx, type:'log'},
  {timestamp: xxx, type:'log'},
  {timestamp: xxx, type:'start'},
  {timestamp: xxx, type:'log'},
  {timestamp: xxx, type:'log'}
]

how do I aggregate the start event and the log event until but not including the next start?

the result should be something like this something like this

{
  events:[
   {timestamp: xxx, type:'start'},
   {timestamp: xxx, type:'log'},
   {timestamp: xxx, type:'log'},
   {timestamp: xxx, type:'log'}
  ]
},
{
  events:[
   {timestamp: xxx, type:'start'},
   {timestamp: xxx, type:'log'},
   {timestamp: xxx, type:'log'}
  ]
}

Answer 1

Assumption: the timestamp field is "sortable" type

You can use $setWindowFields to compute "grouping" for the events. The idea is to find the max timestamp with start in the window range of [unbounded, current] (i.e. find in the documents before and in current document with a $max). After computing the "grouping", just do a simple $group to put the events together in an array.

db.collection.aggregate([
  {
    "$setWindowFields": {
      "sortBy": {
        "timestamp": 1
      },
      "output": {
        "grouping": {
          "$max": {
            "$cond": {
              "if": {
                "$eq": [
                  "$type",
                  "start"
                ]
              },
              "then": "$timestamp",
              "else": -1
            }
          },
          "window": {
            "documents": [
              "unbounded",
              "current"
            ]
          }
        }
      }
    }
  },
  {
    "$group": {
      _id: "$grouping",
      events: {
        $push: {
          timestamp: "$timestamp",
          type: "$type"
        }
      }
    }
  }
])

Mongo Playground

P.S. Your expected result form may suffer from MongoDB 16MB document size if a single grouping contains too many records.