MongoDB: Get most recent objects from embedded arrays and append parent field - mongodb

Question

I have an array of companies where each company has an array of subscription objects. A subscription object has an array of items and a date when those items was added. I want to retrieve only the most recently added subscription object from each company. I also need to be able to identify where each subscription object came from, so I need to append the company name somehow. Sorry for the confusing semantics!
The array of companies:
[
{
"_id": "company1",
"name": "Company 1",
"subscriptions": [
{
"items": [
{
"_id": "5e13b0207b05e37f12f05beb",
"data": 40,
"price": 39
},
{
"_id": "5e13b0207b05e37f12f05bea",
"data": 100,
"price": 59
}
],
"_id": "5e13b0207b05e37f12f05be9",
"date_added": "2019-12-24T22:09:36.442Z"
}
]
},
{ "_id": "company2", "name": "Company 2", "subscriptions": [] },
{
"_id": "company3",
"name": "Company 3",
"subscriptions": [
{
"items": [
{
"_id": "5e13a47d7c370667c7e67d7a",
"data": 40,
"price": 69
},
{
"_id": "5e13a47d7c370667c7e67d79",
"data": 20,
"price": 39
}
],
"_id": "5e13a47d7c370667c7e67d78",
"date_added": "2019-12-24T21:19:57.804Z"
},
{
"items": [
{
"_id": "5e13a4f87c370667c7e67d7d",
"data": 35,
"price": 39
},
{
"_id": "5e13a4f87c370667c7e67d7c",
"data": 60,
"price": 59
}
],
"_id": "5e13a4f87c370667c7e67d7b",
"date_added": "2020-01-04T21:22:00.832Z"
}
]
}
]
Desired output:
[
{
"name": "Company 1",
"items": [
{
"_id": "5e13b0207b05e37f12f05beb",
"data": 40,
"price": 39
},
{
"_id": "5e13b0207b05e37f12f05bea",
"data": 100,
"price": 59
}
],
"_id": "5e13b0207b05e37f12f05be9",
"date_added": "2019-12-24T22:09:36.442Z"
},
{
"name": "Company 3",
"items": [
{
"_id": "5e13a4f87c370667c7e67d7d",
"data": 35,
"price": 39
},
{
"_id": "5e13a4f87c370667c7e67d7c",
"data": 60,
"price": 59
}
],
"_id": "5e13a4f87c370667c7e67d7b",
"date_added": "2020-01-04T21:22:00.832Z"
}
]
What I am trying:
await Companies.aggregate([
{ $unwind: '$subscriptions' },
{ $sort: { 'subscriptions.date_added': -1 } },
{ $group: { _id: '$_id' } }
])
Output:
[{"_id":"company1"},{"_id":"company3"}]
I think I filtered out the correct objects (the most recent ones) but I am only retrieving the _id of the root element, not the nested subscription objects. These id's might be used to identify the subscriptions, but I need also need the actual subscription items.

Answer 1

As all you want is latest subscription object, if that means if it's the last object being pushed to subscriptions array then, Try this :
Companies.aggregate([{$match : {$and :[{subscriptions: {$exists: true}}, {subscriptions: {$ne: []}}]}},
{$addFields :{subscriptions: {$arrayElemAt : ['$subscriptions',-1]} }}, {$project :{_id:0,subscriptions:1, name:1 }}])
Collection data :
/* 1 */
{
"_id" : "company1",
"name" : "Company 1",
"subscriptions" : [
{
"items" : [
{
"_id" : "5e13b0207b05e37f12f05beb",
"data" : 40.0,
"price" : 39.0
},
{
"_id" : "5e13b0207b05e37f12f05bea",
"data" : 100.0,
"price" : 59.0
}
],
"_id" : "5e13b0207b05e37f12f05be9",
"date_added" : "2019-12-24T22:09:36.442Z"
}
]
}
/* 2 */
{
"_id" : "company2",
"name" : "Company 2",
"subscriptions" : []
}
/* 3 */
{
"_id" : "company3",
"name" : "Company 3",
"subscriptions" : [
{
"items" : [
{
"_id" : "5e13a47d7c370667c7e67d7a",
"data" : 40.0,
"price" : 69.0
},
{
"_id" : "5e13a47d7c370667c7e67d79",
"data" : 20.0,
"price" : 39.0
}
],
"_id" : "5e13a47d7c370667c7e67d78",
"date_added" : "2019-12-24T21:19:57.804Z"
},
{
"items" : [
{
"_id" : "5e13a4f87c370667c7e67d7d",
"data" : 35.0,
"price" : 39.0
},
{
"_id" : "5e13a4f87c370667c7e67d7c",
"data" : 60.0,
"price" : 59.0
}
],
"_id" : "5e13a4f87c370667c7e67d7b",
"date_added" : "2020-01-04T21:22:00.832Z"
}
]
}
Result :
/* 1 */
{
"name" : "Company 1",
"subscriptions" : {
"items" : [
{
"_id" : "5e13b0207b05e37f12f05beb",
"data" : 40.0,
"price" : 39.0
},
{
"_id" : "5e13b0207b05e37f12f05bea",
"data" : 100.0,
"price" : 59.0
}
],
"_id" : "5e13b0207b05e37f12f05be9",
"date_added" : "2019-12-24T22:09:36.442Z"
}
}
/* 2 */
{
"name" : "Company 3",
"subscriptions" : {
"items" : [
{
"_id" : "5e13a4f87c370667c7e67d7d",
"data" : 35.0,
"price" : 39.0
},
{
"_id" : "5e13a4f87c370667c7e67d7c",
"data" : 60.0,
"price" : 59.0
}
],
"_id" : "5e13a4f87c370667c7e67d7b",
"date_added" : "2020-01-04T21:22:00.832Z"
}
}

Answer 2

In your solution, you need to accumulate subscriptions with $first operator and with $project you can get desired result.
db.companies.aggregate([
{
$unwind: "$subscriptions"
},
{
$sort: {
"subscriptions.date_added": -1
}
},
{
$group: {
_id: "$_id",
name: {
$first: "$name"
},
subscriptions: {
$first: "$subscriptions"
}
}
},
{
$project: {
_id: "$subscriptions._id",
name: "$name",
date_added: "$subscriptions.date_added",
items: "$subscriptions.items"
}
}
])
MongoPlayground