How to sort by the difference in array contents in MongoDB?

To sort by difference, use aggregate() in MongoDB. Let us create a collection with documents −

> db.demo155.insertOne({"Scores":[{"Value":45},{"Value":50}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e354584fdf09dd6d08539e3")
}
> db.demo155.insertOne({"Scores":[{"Value":60},{"Value":10}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e35458efdf09dd6d08539e4")
}
> db.demo155.insertOne({"Scores":[{"Value":100},{"Value":95}]});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e354599fdf09dd6d08539e5")
}

Display all documents from a collection with the help of find() method −

> db.demo155.find();

This will produce the following output −

{ "_id" : ObjectId("5e354584fdf09dd6d08539e3"), "Scores" : [ { "Value" : 45 }, { "Value" : 50 } ] }
{ "_id" : ObjectId("5e35458efdf09dd6d08539e4"), "Scores" : [ { "Value" : 60 }, { "Value" : 10 } ] }
{ "_id" : ObjectId("5e354599fdf09dd6d08539e5"), "Scores" : [ { "Value" : 100 }, { "Value" : 95 } ] }

Following is the query to sort by the difference in array contents with MongoDB −

> db.demo155.aggregate([
...    { "$match": { "Scores.1": { "$exists": true } } },
...    { "$project": {
...       "Scores": "$Scores",
...       "sub": {
...          "$let": {
...             "vars": {
...                "f": { "$arrayElemAt": [ "$Scores", -2 ] },
...                "l": { "$arrayElemAt": [ "$Scores", -1 ] }
...             },
...             "in": { "$subtract": [ "$$l.Value", "$$f.Value" ] }
...          }
...       }
...    }},
...    { "$sort": { "sub": -1 } }
... ])

This will produce the following output −

{ "_id" : ObjectId("5e354584fdf09dd6d08539e3"), "Scores" : [ { "Value" : 45 }, { "Value" : 50 } ], "sub" : 5 }
{ "_id" : ObjectId("5e354599fdf09dd6d08539e5"), "Scores" : [ { "Value" : 100 }, { "Value" : 95 } ], "sub" : -5 }
{ "_id" : ObjectId("5e35458efdf09dd6d08539e4"), "Scores" : [ { "Value" : 60 }, { "Value" : 10 } ], "sub" : -50 }