Sort Sub Documents in MongoDB

Question

Is there a way to sort sub documents in a mongo query?

For example:

{
  \"_id\" : ObjectId(\"4c69d19532f73ad544000001\"),
  \"content\" : \"blah bla         


        

Answer 1

Starting in Mongo 4.4, the $function aggregation operator allows applying a custom javascript function to implement behaviour not supported by the MongoDB Query Language.

For instance, in order to sort an array of objects by one of their fields:

// {
//   "content": "blah blah blah",
//   "comments": [
//     { "author": "jim", "content": "comment content 1", "date": "07-24-1995" },
//     { "author": "joe", "content": "comment content 2", "date": "07-24-1996" },
//     { "author": "amy", "content": "comment content 3", "date": "09-10-1999" }
//   ]
// }
db.collection.aggregate(
  { $sort: { content: 1 } }, // or whatever sort order
  { $set:
    { "comments":
      { $function: {
          body: function(comments) {
            return comments.sort((a, b) => a.date < b.date);
          },
          args: ["$comments"],
          lang: "js"
      }}
    }
  }
)
// {
//   "content": "blah blah blah",
//   "comments": [
//     { "author": "amy", "content": "comment content 3", "date": "09-10-1999" },
//     { "author": "joe", "content": "comment content 2", "date": "07-24-1996" },
//     { "author": "jim", "content": "comment content 1", "date": "07-24-1995" }
//   ]
// }

$function takes 3 parameters:

• body, which is the function to apply, whose parameter is the array to modify.
• args, which contains the fields from the record that the body function takes as parameter. In our case "$comments".
• lang, which is the language in which the body function is written. Only js is currently available.