Sort by subdocument in MongoDB

To sort by subdocument, use $sort in MongoDB. Let us create a collection with documents −

> db.demo245.insertOne(
...   {
...      "_id": 101,
...      "deatils": [
...         { "DueDate": new ISODate("2019-01-10"), "Value": 45},
...         {"DueDate": new ISODate("2019-11-10"), "Value": 34 }
...      ]
...   }
...);
{ "acknowledged" : true, "insertedId" : 101 }
> db.demo245.insertOne(
...   {
...      "_id": 102,
...      "details": [
...         { "DueDate": new ISODate("2019-12-11"), "Value": 29},
...         {"DueDate": new ISODate("2019-03-10"), "Value":  78}
...      ]
...   }
...);
{ "acknowledged" : true, "insertedId" : 102 }

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

> db.demo245.find();

This will produce the following output −

{
   "_id" : 101, "deatils" : [
      { "DueDate" : ISODate("2019-01-10T00:00:00Z"), "Value" : 45 },
      { "DueDate" : ISODate("2019-11-10T00:00:00Z"), "Value" : 34 }
   ]
}
{
   "_id" : 102, "details" : [
      { "DueDate" : ISODate("2019-12-11T00:00:00Z"), "Value" : 29 },
      { "DueDate" : ISODate("2019-03-10T00:00:00Z"), "Value" : 78 } \
   ] 
}

Following is the query to sort by subdocument −

> db.demo245.aggregate([
...   { "$unwind": "$details" },
...   { "$sort": { "_id": 1, "details.Value": -1 } },
...   { "$group": {
...      "_id": "$_id",
...      "details": { "$push": "$details" }
...   }},
...   { "$sort": { "details.Value": -1 } }
...])

This will produce the following output −

{ "_id" : 102, "details" : [ { "DueDate" : ISODate("2019-03-10T00:00:00Z"), "Value" : 78 }, { "DueDate" : ISODate("2019-12-11T00:00:00Z"), "Value" : 29 } ] }