Transform mongodb array document to a single field - node.js

Question

I have such document structure:
{
favourite_shops: [
{
shop_id: "5961352278cba91cc6a6e8cc",
time: "2017-07-11T14:43:35.465Z"
},
{
shop_id: "5964e446c15c760f9b646d99",
time: "2017-07-11T14:44:40.429Z"
},
{
shop_id: "5964e446c15c760f9b646d98",
time: "2017-07-11T14:44:50.988Z"
}
]
}
How can I transform this to something like this:
{
favourite_shops: [
"5961352278cba91cc6a6e8cc",
"5964e446c15c760f9b646d99",
"5964e446c15c760f9b646d98"
]
}

Answer 1

You can $map on the array itself and get only the fields you need. (This is done within the query itself not after you receive the documents)
example.
given documents;
> db.Shopping.find().pretty()
{
"_id" : ObjectId("59651ce38828356e1a39fde9"),
"favourite_shops" : [
{
"shop_id" : "5961352278cba91cc6a6e8cc",
"time" : "2017-07-11T14:43:35.465Z"
},
{
"shop_id" : "5964e446c15c760f9b646d99",
"time" : "2017-07-11T14:44:40.429Z"
},
{
"shop_id" : "5964e446c15c760f9b646d98",
"time" : "2017-07-11T14:44:50.988Z"
}
]
}
{
"_id" : ObjectId("59665cf3d8145b41e5d2f5da"),
"favourite_shops" : [
{
"shop_id" : "2222",
"time" : "2017-07-11T14:43:35.465Z"
},
{
"shop_id" : "4444",
"time" : "2017-07-11T14:44:40.429Z"
},
{
"shop_id" : "6666",
"time" : "2017-07-11T14:44:50.988Z"
}
]
}
$map on favourite_shops array ({$match} block is optional, you can remove if you want shopping id for all documents)
> db.Shopping.aggregate([[
{
"$match": {
"_id": ObjectId("59651ce38828356e1a39fde9")
}
},
{
"$project": {
"my_favourite_shops": {
"$map": {
"input": "$favourite_shops",
"as": "each_shop",
"in": "$$each_shop.shop_id"
}
}
}
}
]).pretty()
{
"_id" : ObjectId("59651ce38828356e1a39fde9"),
"my_favourite_shops" : [
"5961352278cba91cc6a6e8cc",
"5964e446c15c760f9b646d99",
"5964e446c15c760f9b646d98"
]
}
And, with mongodb 3.4.4, I simply could $project on nested field,
db.Shopping.aggregate([{"$project": {"my_favourite_shops": "$favourite_shops.shop_id"}}]).pretty()
{
"_id" : ObjectId("59651ce38828356e1a39fde9"),
"my_favourite_shops" : [
"5961352278cba91cc6a6e8cc",
"5964e446c15c760f9b646d99",
"5964e446c15c760f9b646d98"
]
}
{
"_id" : ObjectId("59665cf3d8145b41e5d2f5da"),
"my_favourite_shops" : [
"2222",
"4444",
"6666"
]
}

Answer 2

Assuming we have such data structure:
const data = {
favourite_shops: [
{
shop_id: "5961352278cba91cc6a6e8cc",
time: "2017-07-11T14:43:35.465Z"
},
{
shop_id: "5964e446c15c760f9b646d99",
time: "2017-07-11T14:44:40.429Z"
},
{
shop_id: "5964e446c15c760f9b646d98",
time: "2017-07-11T14:44:50.988Z"
}
]
};
Just a bit of ES6 magic:
const result = {
favourite_shops: []
};
data.favourite_shops.forEach(el => result.favourite_shops.push(el.shop_id));
Second elegant approach:
const result = {
favourite_shops: data.favourite_shops.map(el => el.shop_id)
};