Sorting nested Array in MongoDB using aggregate - arrays

Question

I am trying to sort the nested array "subCategoryList on top of sorting by _id on the parent document. Newbie to mongo. Any help will be appreciated
Here is sample documents:
[
{
"_id": 1000,
"name": "Automobiles",
"parentId": "",
"helpText": "Year, Brand, Model, Color, Size"
},
{
"_id": 1004,
"name": "RV / Campers",
"parentId": 1000,
"helpText": ""
},
{
"_id": 1001,
"name": "Car / SUV / Truck",
"parentId": 1000,
"helpText": ""
}
]
Here is what I tried:
db.Category.aggregate([
{
"$match": {
"parentId": ""
}
},
{
"$lookup": {
"from": "Category", "localField": "_id", "foreignField": "parentId", "as": "subCategoryList"
}
},
{
$sort: {
_id: 1
}
}
]).pretty()
But I get:
{
"_id" : 17000,
"name" : "Music",
"parentId" : "",
"helpText" : "Help Text - Brand, Model, Title",
"subCategoryList" : [
{
"_id" : 17001,
"name" : "DVD / Blu-ray",
"parentId" : 17000,
"helpText" : ""
},
{
"_id" : 17002,
"name" : "Player",
"parentId" : 17000,
"helpText" : ""
}
]
}
{
"_id" : 20000,
"name" : "Sports Gear",
"parentId" : "",
"helpText" : "Help Text - Brand, Model, Gear Type, Size, Color,",
"subCategoryList" : [
{
"_id" : 20002,
"name" : "Football",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20007,
"name" : "Tennis",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20008,
"name" : "Cricket",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20004,
"name" : "Hockey",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20003,
"name" : "Golf",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20006,
"name" : "Basketball",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20005,
"name" : "Soccer",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20010,
"name" : "Camping / Hiking",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20009,
"name" : "Cycling",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20001,
"name" : "Baseball",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20012,
"name" : "Skiing",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20011,
"name" : "Swimming",
"parentId" : 20000,
"helpText" : ""
},
{
"_id" : 20099,
"name" : "Other",
"parentId" : 20000,
"helpText" : ""
}
]
}

Answer 1

db.hardwares.aggregate([
{$unwind: "$subCategoryList"},
{$sort: {"subCategoryList._id": 1}},
{$group: {_id:"$_id", subCategoryList: {$push:"$subCategoryList"}}}
]);
It could be helps you, try like this...
May be it didn't work but it gives an idea

Answer 2

Mongo doesn't have a built in function to sort an inner array, However Mongo v4.4 introduces the $function aggregation operator. this allows us to use custom javascript functions within a pipeline.
You could use it like so:
db.Category.aggregate([
{
$addFields: {
"subCategoryList":
{
$function: {
body: function (categories) {
return categories.sort((a, b) => a._id - b._id);
},
args: ["$subCategoryList"],
lang: "js"
}
}
}
}
])
For lesser Mongo versions you will have to first $unwind the array. then $sort it and finally constructing the original structure or in your case because you are $lookuping the inner array you could use the other $lookup syntax introduced at v3.6 to $sort within the $lookup:
db.Category.aggregate([
{
"$match": {
"parentId": ""
}
},
{
"$lookup": {
"from": "Category",
let: {id: "$_id"},
pipeline: [
{
$match: {
$expr: {
$eq: ["$$id", "$parentId"]
}
}
},
{
$sort: {
_id: 1
}
}
],
"as": "subCategoryList"
}
},
{
$sort: {
_id: 1
}
}
]);