Query to get a value by subtracting a value from current and next document - mongodb

Question

I have a mongo db collection like below,
{
"id": ObjectId("132456"),
reading :[
{
"weight" : {
"measurement" : 82.0,
"unit" : "kg"
}
}
],
"date" : ISODate("2018-09-12T11:45:08.174Z")
},
{
"id": ObjectId("132457"),
reading :[
{
"weight" : {
"measurement" : 80.0,
"unit" : "kg"
}
}
],
"date" : ISODate("2018-09-12T10:45:08.174Z")
},
{
"id": ObjectId("132458"),
reading :[
{
"weight" : {
"measurement" : 85.0,
"unit" : "kg"
}
}
],
"date" : ISODate("2018-09-11T09:45:08.174Z")
}
I need a mongo db query that will give me the current weight and the weight difference between the current and next record.
Example output below,
{
"id": ObjectId("132456"),
"currentWeight": 75.0,
"weightDifference": 2.0,
"date" : ISODate("2018-09-12T11:45:08.174Z")
},
{
"id": ObjectId("132457"),
"currentWeight": 80.0,
"weightDifference": -5.0,
"date" : ISODate("2018-09-12T10:45:08.174Z")
}
I was not able to get the weight from next document to subtract the weight from current document.
Thanks in advance for your help
My try for the above problem,
db.measurementCollection.aggregate([
{
$match : { "date" : { $gte : new ISODate("2018-09-01T00:00:00.000Z") , $lte : new ISODate("2018-09-12T23:59:59.000Z") } }
},
{
$project : { "date" : 1 ,
"currentWeight" : {$arrayElemAt: [ "$reading.weight.measurement", 0 ]}
},
{ $sort: {"date":-1} },
{
$addFields : {
"weigtDifference" :
{
{
$limit: 2
},
{
$group: {
_id: null,
'count1': {$first: '$currentWeight'},
'count2': {$last: '$currentWeight'}
}
},
{
$subtract: ['$count1', '$count2']
}
}
}
}
])

Answer 1

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" }}
])