Find latest record based on Group Mongoose

Question

I'm quite new to NoSQL and struggling to write this query I am using Mongoose on Node.js

What I want to achieve is to get ONE latest result based on Group of Device ID. I have no trouble writing this in SQL but having hard time doing it in NoSQL.

Here is the Model setup

_id     DeviceID     Coordinate:{lat, long}
1       2            lat: 1, long: 2
2       3            lat: 2, long: 3
3       1            lat: 3, long: 3
4       3            lat: 5, long: 4
5       2            lat: 7, long: 5
6       2            lat: 9, long: 6
7       3            lat: 111, long: 7
8       2            lat: 113, long: 8

The output that I want is:

_id     DeviceID     Coordinate:{lat, long}
3       1            lat: 3, long: 3
7       3            lat: 111, long: 7
8       2            lat: 113, long: 8

This is what I have tried but the result I've got is undefined

Note: beginDayID, endDayID are variables of mongoose ObjectId represents the _id of beginning and end of day.

mongoose.model('GPSData').aggregate([
  {$match: {_id:{$gte: beginDayID, $lt: endDayID}}},
  {$unwind: "$Coordinates"},
  {$project: {DeviceID: '$DeviceID' }},
  {$group: { DeviceID: '$DeviceID', $lat: '$Coordinates.lat', $long: '$Coordinates.long'}}

  ], (e, data) => {
     console.error(e)
     console.log(data)
     if (e) return callback(e, null);
     return callback(null, data);
   })

Answer 1

I assume you have documents somewhat similar to this

/* 1 */
{
    "_id" : 1,
    "DeviceID" : 1,
    "Coordinate" : {
        "lat" : 1,
        "long" : 2
    }
}

/* 2 */
{
    "_id" : 2,
    "DeviceID" : 2,
    "Coordinate" : {
        "lat" : 1,
        "long" : 6
    }
}
...

then an aggregation pipeline like this should work

mongoose.model('GPSData').aggregate([
  {
      $match: ... // your match filter criteria
  },
  {
      $sort: {
          _id: 1
      }
  },
  { 
      $group: { 
          _id: '$DeviceID',
          lastId: { $last: '$_id' },
          lat: { $last: '$Coordinate.lat' }, 
          long: { $last:'$Coordinate.long' }
      }
  },
  {
      $project: {
          _id: '$lastId',
          DeviceID: '$_id',
          lat: 1,
          long: 1
      }
  }
])

The output documents' shape look like this

/* 1 */
{
    "_id" : 1,
    "DeviceID" : 1,
    "Coordinate" : {
        "lat" : 1,
        "long" : 2
    }
}

/* 2 */
{
    "_id" : 2,
    "DeviceID" : 2,
    "Coordinate" : {
        "lat" : 1,
        "long" : 6
    }
}

Note the additional stage of $sort as you have to specify an order when talking about keeping the 'last values'. You may have to specify another sorting if you have other requirements