Find a nested object field inside an array in mongodb aggregate - mongodb

Question

I have this object as below.
{
"_id" : ObjectId("5ec80a981e89a84b19934039"),
"status" : "active",
"organizationId" : "1",
"productId" : "1947",
"name" : "BOOKEND & PAPER WEIGHT SET – ZODIAC PIG – RED COPPER + PLATINUM",
"description" : "This global exclusive Zodiac bookend and paperweight set from Zuny will stand auspiciously on your bookcase and table, spreading good luck and fortune throughout your home just in time for the Year of the Pig.",
"brand" : "ZUNY",
"created" : "2018-09-28 00:00:00",
"updated" : "2020-05-22 09:19:07",
"mainImage" : "https://",
"availableOnline" : true,
"colors" : [
{
"images" : [
{
"type" : "studio",
"url" : "https://"
},
{
"type" : "studio",
"url" : "https://"
},
{
"type" : "studio",
"url" : "https://"
}
],
"extraInfo" : [
{
"type" : "text-tag",
"title" : "CATEGORY",
"tags" : [
"HOME FURNISHING & DÉCOR",
"LIFESTYLE"
]
},
{
"type" : "text-tag",
"title" : "BRAND",
"tags" : [
"ZUNY"
]
},
{
"type" : "text-tag",
"title" : "COLOUR",
"tags" : [
"GOLD",
"ROSE GOLD"
]
},
{
"type" : "text-tag",
"title" : "SEASON",
"tags" : [
"AW(2018)"
]
},
{
"type" : "text-tag",
"title" : "HASHTAG",
"tags" : [
"BOOKCASES",
"BOOKEND",
"COLOUR",
"EXCLUSIVE",
"GLOBAL EXCLUSIVE",
"HOME",
"LEATHER",
"MOTIF",
"OBJECTS",
"PAPER",
"PAPERWEIGHT",
"PLATINUM",
"SET",
"SYNTHETIC",
"ZODIAC",
"HANDMADE",
"time"
]
}
],
"_id" : ObjectId("5ec80a981e89a84b1993403a"),
"colorId" : "1",
"color" : "ROSE GOLD",
"status" : "active",
"sizes" : [
{
"extraInfo" : [
{
"type" : "text-block",
"title" : "Size And Fit",
"text" : ""
},
{
"type" : "text-block",
"title" : "Information",
"text" : "Global exclusive. Colour: Copper/Platinum. Set includes: Zodiac Pig bookend (x 1), Zodiac Pig paperweight (x 1). Metallic copper- and platinum-tone synthetic leather. Pig motif. Iron pellet filling. Handmade"
}
],
"_id" : ObjectId("5ec80a981e89a84b1993403b"),
"sizeId" : "1",
"neo" : "0210111790664",
"size" : "*",
"originalPrice" : "1060.00",
"sellingPrice" : "1060.00",
"discountPercent" : "0.00",
"url" : "https://",
"status" : "active",
"currency" : "HK$",
"stores" : [
{
"storeId" : "1",
"quantity" : 70,
"_id" : ObjectId("5ec80a981e89a84b1993403c"),
"available" : 70,
"reserved" : 0,
"name" : "Park Street",
"status" : "active"
},
{
"storeId" : "2",
"quantity" : 95,
"_id" : ObjectId("5ec80a981e89a84b1993403d"),
"name" : "Rashbehari",
"status" : "active"
}
]
}
]
}
],
"__v" : 0
}
I want the output as follows
{
"name": "Mock Collection",
"collectionId": "92",
"products": [
{
"title": "GLOBAL EXCLUSIVE OFF-SHOULDER SHIRT DRESS",
"imageUrl": "https://",
"productId": "21174",
"currency": "" // This should be this.colors[0].sizes[0].currency
},
]
}
How to get the nested field. I tried using arrayElemAt by which I was able to get to colors[0]. But I am confused how to get inside the nested object of sizes from there. Also the currency node should have the exact value. It comes like currency:{currency: value} which I don't want.
Please help!

Answer 1

Not sure how you've got that output but to extract currency from first object of sizes then you need to try this :
db.collection.aggregate([
{
$project: {
currency: {
$arrayElemAt: [
{
$arrayElemAt: [ "$colors.sizes.currency", 0 ] // gives an array of currency values, in your case since you've only one object just an array of one value
},
0
]
}
}
}
])
Test : mongoplayground