Count by elements in list and by field - mongodb

Question

I have a MongoDB collection that looks like this:
{ "_id" : 1, "owner" : "Alice", airline: "RSAirlines", "content" : ["shoes", "pants", "sockets"]}
{ "_id" : 2, "owner" : "Bob", airline: "RSAirlines", "content" : ["phone", "pants"]}
{ "_id" : 3, "owner" : "Charlie", airline: "RSAirlines", "content" : ["shoes", "pants", "bag"]}
{ "_id" : 4, "owner" : "Mary" ,airline: "AirES" "content" : ["sandals", "coins", "sockets"]}
{ "_id" : 5, "owner" : "Olivia", airline: "AirES", "content" : ["gloves", "pants", "sockets"]}
{ "_id" : 6, "owner" : "Dan", airline: "AirES", "content" : ["sockets", "wallet"]}
{ "_id" : 7, "owner" : "Erin", airline: "AirES", "content" : ["pants", "sockets", "dress"]}
I would like to aggregate them to get the following results:
{ "_id": "RSAirlines", "counts": {
"shoes": 2,
"pants": 3,
"sockets": 1,
"phone": 1,
"bag": 1
}}
{ "_id": "AirES", "counts": {
"sandals": 1,
"coins": 1,
"sockets": 4,
"wallet": 1,
"dress": 1,
"pants": 2
}}
Previous I saw this answer for counting the elements, but now I would like to count them by airline.

Answer 1

$unwind deconstruct content array
$group by airline and content and get the total count
$group by the only airline and construct counts array key-value format
$arrayToObject convert key-value array to object
db.collection.aggregate([
{ $unwind: "$content" },
{
$group: {
_id: {
airline: "$airline",
content: "$content"
},
count: { $sum: 1 }
}
},
{
$group: {
_id: "$_id.airline",
counts: {
$push: {
k: "$_id.content",
v: "$count"
}
}
}
},
{ $project: { counts: { $arrayToObject: "$counts" } } }
])
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