Dynamic keys from values in MongoDB - mongodb

Question

Say I have this:
{
"_id" : "ENVD",
"years" : [
{
"year" : "2013",
"avgInstructor" : 5.144999999999998
},
{
"year" : "2012",
"avgInstructor" : 5.194436090225564
}
]
}
I need to be able to find the difference in the avgInstructor field from 2012-13. I was thinking I could transform the keys somehow using a $project which would make the year be the key, and the avgInstructor rating be the value. So it would look like this:
{
"_id" : "ENVD",
"years" : {
"2013" : 5.144999999999998,
"2012" : 5.194436090225564
}
}
Is this possible? Keep in mind my main goal is to be able to run a subtraction like this pseudocode : years['2013'].avgInstructor - years['2013'].avgInstructor. So if you see an easier way, that would be great as well. I am not sure of the best way to go about this in the context of an aggregation pipeline. Can someone help?

Answer 1

For people that ends up here looking for a solution to transform an array to an object using aggregation in a more recent version of MongoDB:
MongoDB 3.4.4 introduced $arrayToObject
You have to $map years
{
$set: {
years: {
$map: {
input: "$years",
as: "year",
in: [
"$$year.year",
"$$year.avgInstructor"
]
}
}
}
}
to be like this
{
"_id" : "ENVD",
"years": [
["2013", 5.144999999999998],
["2012", 5.194436090225564]
]
}
then use $arrayToObject
{
$set: {
years: {
$arrayToObject: "$years"
}
}
}
Or combine the two steps together
{
$set: {
years: {
$arrayToObject: {
$map: {
input: "$years",
as: "year",
in: [
"$$year.year",
"$$year.avgInstructor"
]
}
}
}
}
}

Answer 2

A Possible answer ...
Unwind first
project to make it a little easier to handle
Sort on _id and then on year
group by _id to pickup the first and the last value
final projection to get subtracted value
db.coll.aggregate ( [
{ "$unwind" : "$years" } ,
{ $project : { "year" : "$years.year", "avgInstructor" : "$years.avgInstructor" } },
{ $sort : { "_id" : 1, "year" : 1 } },
{ $group : { "_id" : "$_id", "val_min" : { $first : "$avgInstructor" }, "val_max" : { $last : "$avgInstructor" } } },
{ $project : { "diff" : { $subtract : [ "$val_min", "$val_max" ] } } }
] )