How to calculate difference between values of different documents using mongo aggregation? - mongodb

Question

Hi my mongo structure as below
{
"timemilliSec":1414590255,
"data":[
{
"x":23,
"y":34,
"name":"X"
},
{
"x":32,
"y":50,
"name":"Y"
}
]
},
{
"timemilliSec":1414590245,
"data":[
{
"x":20,
"y":13,
"name":"X"
},
{
"x":20,
"y":30,
"name":"Y"
}
]
}
Now I want to calculate difference of first document and second document and second to third in this way
so calculation as below
diffX = ((data.x-data.x)/(data.y-data.y)) in our case ((23-20)/(34-13))
diffY = ((data.x-data.x)/(data.y-data.y)) in our case ((32-20)/(50-30))

Answer 1

Tough question in principle, but I'm going to stay with the simplified case you present of two documents and base a solution around that. The concepts should abstract, but are more difficult for expanded cases. Possible with the aggregation framework in general:
db.collection.aggregate([
// Match the documents in a pair
{ "$match": {
"timeMilliSec": { "$in": [ 1414590255, 1414590245 ] }
}}
// Trivial, just keeping an order
{ "$sort": { "timeMilliSec": -1 } },
// Unwind the arrays
{ "$unwind": "$data" },
// Group first and last
{ "$group": {
"_id": "$data.name",
"firstX": { "$first": "$data.x" },
"lastX": { "$last": "$data.x" },
"firstY": { "$first": "$data.y" },
"lastY": { "$last": "$data.y" }
}},
// Difference on the keys
{ "$project": {
"diff": {
"$divide": [
{ "$subtract": [ "$firstX", "$lastX" ] },
{ "$subtract": [ "$firstY", "$lastY" ] }
]
}
}},
// Not sure you want to take it this far
{ "$group": {
"_id": null,
"diffX": {
"$min": {
"$cond": [
{ "$eq": [ "$_id", "X" ] },
"$diff",
false
]
}
},
"diffY": {
"$min": {
"$cond": [
{ "$eq": [ "$_id", "Y" ] },
"$diff",
false
]
}
}
}}
])
Possibly overblown, not sure of the intent, but the output of this based on the sample would be:
{
"_id" : null,
"diffX" : 0.14285714285714285,
"diffY" : 0.6
}
Which matches the calculations.
You can adapt to your case, but the general principle is as shown.
The last "pipeline" stage there is a little "extreme" as all that is done is combine the results into a single document. Otherwise, the "X" and "Y" results are already obtained in two documents in the pipeline. Mostly by the $group operation with $first and $last operations to find the respective elements on the grouping boundary.
The subsequent operations in $project as a pipeline stage performs the required math to determine the distinct results. See the aggregation operators for more details, particularly $divide and $subtract.
Whatever you do you follow this course. Get a "start" and "end" pair on your two keys. Then perform the calculations.