"query to get a value by subtracting a value from current and next document" Code Answer

You can try below aggregation but I will not recommend you to use this with the large data set.

db.collection.aggregate([
  { "$match": {
    "date" : {
      "$gte": new ISODate("2018-09-01T00:00:00.000Z"),
      "$lte": new ISODate("2018-09-12T23:59:59.000Z")
    }
  }},
  { "$unwind": "$reading" },
  { "$sort": { "date": -1 }},
  { "$group": { "_id": null, "data": { "$push": "$$ROOT" }}},
  { "$project": {
      "data": {
        "$filter": {
          "input": {
            "$map": {
              "input": { "$range": [0, { "$size": "$data" }] },
              "as": "tt",
              "in": {
                "$let": {
                  "vars": {
                    "first": { "$arrayElemAt": ["$data", "$$tt"] },
                    "second": { "$arrayElemAt": ["$data", { "$add": ["$$tt", 1] }] }
                  },
                  "in": {
                    "currentWeight": "$$first.reading.weight.measurement",
                    "weightDifference": { "$subtract": ["$$second.reading.weight.measurement", "$$first.reading.weight.measurement"] },
                    "_id": "$$first._id",
                    "date": "$$first.date"
                  }
                }
              }
            }
          },
          "cond": { "$ne": ["$$this.weightDifference", null] }
        }
      }
    }
  },
  { "$unwind": "$data" },
  { "$replaceRoot": { "newRoot": "$data" }}
])

By ScientiaEtVeritas on June 23 2022