How to match inside $lookup used of MongoDB aggregation? - node.js

Question

This is the query I've been using for getting some random uniqueXperiences, in that I've lookup xpert and I want to project its _id and name.
db.getCollection('uniquexperiences').aggregate([{
'$sample': {
'size': 2
}
}, {
'$lookup': {
'from': 'users',
'as': 'Xpert',
'pipeline':[{
'$project': {
'name': 1
}
}]
}
}]);
The output I'm getting is something like this:
/* 1 */
{
"_id" : ObjectId("5b558b5997199d3ec4953e6b"),
"updatedBy" : ObjectId("5b433dbf6a93e56a21f610eb"),
"updatedAt" : ISODate("2018-10-10T10:56:36.952Z"),
"createdBy" : ObjectId("5b433dbf6a93e56a21f610eb"),
"createdAt" : ISODate("2018-07-23T08:01:29.125Z"),
"title" : "Horse riding at EIRS",
"organisationFlag" : false,
"xperienceSubCategory" : [
ObjectId("5b4de8199f07bf0b303b8c5f")
],
"xpert" : [
ObjectId("5b51b054054a962cc6f914db")
],
"description" : "Beautiful rugged landscape makes Ladakh stand apart in the country. There is indeed so much that needs to be explored here and little did we know that amongst the best ways to explore this place is riding a horse. In the remote places like Zanskar Valley, where paved roads are rare and motor biking is not every ones cup of tea, horses make an incredible means of travel. One can access to Zangla, Penzela Pass, Panikhar Fort and Zongkhul, Stongdey and Gelugpa monasteries on a horseback.",
"__v" : 4,
"images" : [
{
"public_id" : "dotttg3xm0vvqygr4n1w",
"version" : 1537776979,
"signature" : "4fb562aa4115941e78f773dd11ed6be37a337acd",
"width" : 1280,
"height" : 640,
"format" : "jpg",
"resource_type" : "image",
"url" : "http://res.cloudinary.com/xplr-qa/image/upload/v1537776979/dotttg3xm0vvqygr4n1w.jpg",
"secure_url" : "https://res.cloudinary.com/xplr-qa/image/upload/v1537776979/dotttg3xm0vvqygr4n1w.jpg",
"_id" : ObjectId("5ba89d55ac69b4784a4f3a1f")
},
{
"public_id" : "cfdepst0r2wz39rowvzd",
"version" : 1537776980,
"signature" : "e770b7354c01ffce61c2e3cb83c79456dacc5438",
"width" : 2100,
"height" : 1526,
"format" : "jpg",
"resource_type" : "image",
"url" : "http://res.cloudinary.com/xplr-qa/image/upload/v1537776980/cfdepst0r2wz39rowvzd.jpg",
"secure_url" : "https://res.cloudinary.com/xplr-qa/image/upload/v1537776980/cfdepst0r2wz39rowvzd.jpg",
"_id" : ObjectId("5ba89d55ac69b4784a4f3a1e")
},
{
"public_id" : "lqfk6twitfefrjq1h2jq",
"version" : 1537776979,
"signature" : "ad1e0079a9419bde0047228446be17218117cf2e",
"width" : 2500,
"height" : 1666,
"format" : "jpg",
"resource_type" : "image",
"url" : "http://res.cloudinary.com/xplr-qa/image/upload/v1537776979/lqfk6twitfefrjq1h2jq.jpg",
"secure_url" : "https://res.cloudinary.com/xplr-qa/image/upload/v1537776979/lqfk6twitfefrjq1h2jq.jpg",
"_id" : ObjectId("5ba89d55ac69b4784a4f3a1d")
}
],
"source" : "",
"price" : 400,
"meetingLocation" : [],
"coordinates" : [
72.955162,
19.172098
],
"purchaseCount" : 10,
"Xpert" : [
{
"_id" : ObjectId("5b518101752982066a0dcc08"),
"name" : {
"last" : "Bourdain",
"first" : "Anthony"
}
},
{
"_id" : ObjectId("5b51b054054a962cc6f914db"),
"name" : {
"last" : "Yane",
"first" : "Sid"
}
},
{
"_id" : ObjectId("5b51c00597199d3ec4953e68"),
"name" : {
"last" : "ipsum",
"first" : "Lorem"
}
},
{
"_id" : ObjectId("5b59a8cdca722c3177cf3e63"),
"name" : {
"last" : "Lorem",
"first" : "Test"
}
},
{
"_id" : ObjectId("5b5eea6e2d4b6a04b2d9d599")
},
{
"_id" : ObjectId("5b5f02e22d4b6a04b2d9d5f1")
},
{
"_id" : ObjectId("5b60364f9e866d6453fef3ea"),
"name" : {
"last" : "Lorem",
"first" : "ipsum"
}
},
{
"_id" : ObjectId("5b6d70f96aaf19087ac6d9e9"),
"name" : {
"first" : "Indrajit",
"last" : "Jadhav"
}
},
{
"_id" : ObjectId("5b6d81953da5c50974642974"),
"name" : {
"last" : "Lorem",
"first" : "shin"
}
},
{
"_id" : ObjectId("5b6d831f3da5c50974642975"),
"name" : {
"last" : "Lorem",
"first" : "Shin "
}
},
{
"_id" : ObjectId("5b6d83b83da5c50974642976"),
"name" : {
"last" : "wert",
"first" : "fadsg"
}
},
{
"_id" : ObjectId("5b6d83ee3da5c50974642977"),
"name" : {
"last" : "asdfsaf",
"first" : "sadf"
}
},
{
"_id" : ObjectId("5b7ba46cd0e5fa137805552b"),
"name" : {
"first" : "Vivek",
"last" : "Pandey"
}
},
{
"_id" : ObjectId("5b7bb359d0e5fa13780555a9"),
"name" : {
"first" : "theamalageeks",
"last" : "null"
}
},
{
"_id" : ObjectId("5b7bd19ed0e5fa137805565d"),
"name" : {
"first" : "Anikesh",
"last" : "Baburanjan"
}
},
{
"_id" : ObjectId("5b7fa359211f5649518ae311"),
"name" : {
"first" : "Akhsay",
"last" : "Gaikwad"
}
},
{
"_id" : ObjectId("5b80181fab8747537011be5c"),
"name" : {
"first" : "Sagar",
"last" : "Bisen"
}
},
{
"_id" : ObjectId("5b83f1c5de663c0dcccfb483"),
"name" : {
"first" : "INDRAJIT",
"last" : "JADHAV"
}
},
{
"_id" : ObjectId("5b92154efc790f18121f9e02"),
"name" : {
"first" : "sagar",
"last" : "bisen"
}
},
{
"_id" : ObjectId("5ba39c509350135c9951ecb6"),
"name" : {
"first" : "Anikesh",
"last" : "Baburajan"
}
},
{
"_id" : ObjectId("5ba3b8e79350135c9951ecb7"),
"name" : {
"first" : "Anurag",
"last" : "Banerjee"
}
},
{
"_id" : ObjectId("5ba3c14e9350135c9951ecb8"),
"name" : {
"first" : "Dave",
"last" : "Jhala"
}
},
{
"_id" : ObjectId("5ba4dfcf6bb5277e1d7db2af"),
"name" : {
"first" : "Mark",
"last" : "Henry"
}
},
{
"_id" : ObjectId("5ba4e1c0452d7e01ce3dd34e"),
"name" : {
"first" : "Ankita",
"last" : "Mestry"
}
},
{
"_id" : ObjectId("5ba7795b452d7e01ce3dd34f"),
"name" : {
"first" : "Abhishek",
"last" : "Gupta"
}
},
{
"_id" : ObjectId("5ba9ec5844996a4be81a0c12"),
"name" : {
"first" : "Neeti",
"last" : "Patil"
}
},
{
"_id" : ObjectId("5baa1abf44996a4be81a0c13"),
"name" : {
"first" : "Krishna",
"last" : "S"
}
},
{
"_id" : ObjectId("5bb481d4c77dd914de2cb2f7"),
"name" : {
"first" : "krishna",
"last" : "shetty"
}
}
]
}
/* 2 */
{
"_id" : ObjectId("5bab23c897f85d07b9235a19"),
"updatedBy" : ObjectId("5b433dbf6a93e56a21f610eb"),
"updatedAt" : ISODate("2018-10-10T10:57:01.790Z"),
"createdBy" : ObjectId("5b433dbf6a93e56a21f610eb"),
"createdAt" : ISODate("2018-09-26T06:14:32.637Z"),
"xpert" : ObjectId("5ba4dfcf6bb5277e1d7db2af"),
"title" : "Mumbai Temples Tour",
"images" : [
{
"public_id" : "uvmemwfwmjfl7pmv4upk",
"version" : 1537943055,
"signature" : "5358320e4f396a2ee980e767a16d4a92228c95d6",
"width" : 1440,
"height" : 900,
"format" : "jpg",
"resource_type" : "image",
"url" : "http://res.cloudinary.com/xplr-qa/image/upload/v1537943055/uvmemwfwmjfl7pmv4upk.jpg",
"secure_url" : "https://res.cloudinary.com/xplr-qa/image/upload/v1537943055/uvmemwfwmjfl7pmv4upk.jpg",
"_id" : ObjectId("5bab260f97f85d07b9235a20")
},
{
"public_id" : "zztqwdbl7ejccxmougad",
"version" : 1537943055,
"signature" : "bcb413431d518b243d801f92ff9cf0a8f9bd7682",
"width" : 1920,
"height" : 1080,
"format" : "jpg",
"resource_type" : "image",
"url" : "http://res.cloudinary.com/xplr-qa/image/upload/v1537943055/zztqwdbl7ejccxmougad.jpg",
"secure_url" : "https://res.cloudinary.com/xplr-qa/image/upload/v1537943055/zztqwdbl7ejccxmougad.jpg",
"_id" : ObjectId("5bab260f97f85d07b9235a1f")
}
],
"organisationFlag" : false,
"xperienceSubCategory" : [
ObjectId("5b616b4c9e866d6453fef3eb")
],
"description" : "Global Vipassana Pagoda is an expression of our gratitude towards the Buddha who strived for incalculable aeons to reach Supreme Enlightenment.",
"__v" : 4,
"price" : 800,
"source" : "",
"meetingLocation" : [],
"coordinates" : [
72.955162,
19.172098
],
"primaryLocation" : ObjectId("5bacb0192e0b2751f0f7f057"),
"purchaseCount" : 20,
"Xpert" : [
{
"_id" : ObjectId("5b518101752982066a0dcc08"),
"name" : {
"last" : "Bourdain",
"first" : "Anthony"
}
},
{
"_id" : ObjectId("5b51b054054a962cc6f914db"),
"name" : {
"last" : "Yane",
"first" : "Sid"
}
},
{
"_id" : ObjectId("5b51c00597199d3ec4953e68"),
"name" : {
"last" : "ipsum",
"first" : "Lorem"
}
},
{
"_id" : ObjectId("5b59a8cdca722c3177cf3e63"),
"name" : {
"last" : "Lorem",
"first" : "Test"
}
},
{
"_id" : ObjectId("5b5eea6e2d4b6a04b2d9d599")
},
{
"_id" : ObjectId("5b5f02e22d4b6a04b2d9d5f1")
},
{
"_id" : ObjectId("5b60364f9e866d6453fef3ea"),
"name" : {
"last" : "Lorem",
"first" : "ipsum"
}
},
{
"_id" : ObjectId("5b6d70f96aaf19087ac6d9e9"),
"name" : {
"first" : "Indrajit",
"last" : "Jadhav"
}
},
{
"_id" : ObjectId("5b6d81953da5c50974642974"),
"name" : {
"last" : "Lorem",
"first" : "shin"
}
},
{
"_id" : ObjectId("5b6d831f3da5c50974642975"),
"name" : {
"last" : "Lorem",
"first" : "Shin "
}
},
{
"_id" : ObjectId("5b6d83b83da5c50974642976"),
"name" : {
"last" : "wert",
"first" : "fadsg"
}
},
{
"_id" : ObjectId("5b6d83ee3da5c50974642977"),
"name" : {
"last" : "asdfsaf",
"first" : "sadf"
}
},
{
"_id" : ObjectId("5b7ba46cd0e5fa137805552b"),
"name" : {
"first" : "Vivek",
"last" : "Pandey"
}
},
{
"_id" : ObjectId("5b7bb359d0e5fa13780555a9"),
"name" : {
"first" : "theamalageeks",
"last" : "null"
}
},
{
"_id" : ObjectId("5b7bd19ed0e5fa137805565d"),
"name" : {
"first" : "Anikesh",
"last" : "Baburanjan"
}
},
{
"_id" : ObjectId("5b7fa359211f5649518ae311"),
"name" : {
"first" : "Akhsay",
"last" : "Gaikwad"
}
},
{
"_id" : ObjectId("5b80181fab8747537011be5c"),
"name" : {
"first" : "Sagar",
"last" : "Bisen"
}
},
{
"_id" : ObjectId("5b83f1c5de663c0dcccfb483"),
"name" : {
"first" : "INDRAJIT",
"last" : "JADHAV"
}
},
{
"_id" : ObjectId("5b92154efc790f18121f9e02"),
"name" : {
"first" : "sagar",
"last" : "bisen"
}
},
{
"_id" : ObjectId("5ba39c509350135c9951ecb6"),
"name" : {
"first" : "Anikesh",
"last" : "Baburajan"
}
},
{
"_id" : ObjectId("5ba3b8e79350135c9951ecb7"),
"name" : {
"first" : "Anurag",
"last" : "Banerjee"
}
},
{
"_id" : ObjectId("5ba3c14e9350135c9951ecb8"),
"name" : {
"first" : "Dave",
"last" : "Jhala"
}
},
{
"_id" : ObjectId("5ba4dfcf6bb5277e1d7db2af"),
"name" : {
"first" : "Mark",
"last" : "Henry"
}
},
{
"_id" : ObjectId("5ba4e1c0452d7e01ce3dd34e"),
"name" : {
"first" : "Ankita",
"last" : "Mestry"
}
},
{
"_id" : ObjectId("5ba7795b452d7e01ce3dd34f"),
"name" : {
"first" : "Abhishek",
"last" : "Gupta"
}
},
{
"_id" : ObjectId("5ba9ec5844996a4be81a0c12"),
"name" : {
"first" : "Neeti",
"last" : "Patil"
}
},
{
"_id" : ObjectId("5baa1abf44996a4be81a0c13"),
"name" : {
"first" : "Krishna",
"last" : "S"
}
},
{
"_id" : ObjectId("5bb481d4c77dd914de2cb2f7"),
"name" : {
"first" : "krishna",
"last" : "shetty"
}
}
]
}
Instead of getting multiple Xperts in Xpert
Now I want to match the Xpert fields _id with xpert field of the output

Answer 1

Use the below aggregation.
Use $lookup pipeline variant. Let expression checks for a array type for xpert and converts into array when it is not. $match to collect all the matching users documents. $project to output name from user document.
db.uniquexperiences.aggregate([
{"$sample":{"size":2}},
{"$lookup":{
"from":"users",
"let":{"xpert":{"$cond":[{"$isArray":"$xpert"},"$xpert",["$xpert"]]}},
"pipeline":[
{"$match":{"$expr":{"$in":["$_id","$$xpert"]}}},
{"$project":{"name":1}}],
"as":"Xpert"
}}
])