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

To sort by the difference in array contents, use MongoDB's aggregation pipeline with $project to calculate differences and $sort to order results. This is useful when comparing values between array elements.

Syntax

db.collection.aggregate([
    { $project: { 
        "fieldName": "$fieldName",
        "difference": { 
            $subtract: [
                { $arrayElemAt: ["$arrayField", index1] },
                { $arrayElemAt: ["$arrayField", index2] }
            ]
        }
    }},
    { $sort: { "difference": 1 } }
]);

Sample Data

db.demo155.insertMany([
    {"Scores": [{"Value": 45}, {"Value": 50}]},
    {"Scores": [{"Value": 60}, {"Value": 10}]},
    {"Scores": [{"Value": 100}, {"Value": 95}]}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e354584fdf09dd6d08539e3"),
        ObjectId("5e35458efdf09dd6d08539e4"), 
        ObjectId("5e354599fdf09dd6d08539e5")
    ]
}

Example: Sort by Difference Between Last Two Elements

Calculate the difference between the last two array elements and sort in descending order ?

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 } }
]);

{ "_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 }

How It Works

• $match ensures documents have at least 2 array elements
• $arrayElemAt with -2 and -1 gets the second-to-last and last elements
• $let creates variables to store array elements for cleaner subtraction
• $subtract calculates the difference: last_value - second_last_value

Conclusion

Use aggregation with $arrayElemAt and $subtract to calculate differences between array elements. The $sort stage then orders documents by these computed differences.