Dynamic keys from values in MongoDB

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"            ]          }        }      }    }  }

A Possible answer ...

1. Unwind first
2. project to make it a little easier to handle
3. Sort on _id and then on year
4. group by _id to pickup the first and the last value

5. 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" ] }  } }] )