How to $lookup in nested array of objects - mongodb

Question

I have to two collections one is tours and other is destinations so in tours have i have an array of locations which has destination object with an id and that id is belongs to another destinations collection but the thing is i am not be able to lookup details of destination in the array of locations. every tried many query search here too. but not getting expected result.
Tours :
{
"_id" : ObjectId("5f3122f4d8d57e3b9650e5b4"),
"title" : "tour 1",
"locations" : [
{
"destination" : {
"id" : "5ec5ae9037ea99f20a79071a"
},
"services" : {
"hotel" : true
}
},
{
"destination" : {
"id" : "5ec5ae8e37ea99f20a78ef8c"
},
"services" : {
"hotel" : true
}
}
]
}
{
"_id" : ObjectId("5f2d65e68bc6e9155310d147"),
"title" : "tour 2",
"locations" : [
{
"destination" : {
"id" : "5ecf994435c3a6025d5bf126"
},
"services" : {
"hotel" : true
}
}
]
}
{
"_id" : ObjectId("5f2d66398bc6e9155310d161"),
"title" : "tour 3",
"locations" : [
{
"destination" : {
"id" : "5ec5ae8e37ea99f20a78ef8d"
},
"services" : {
"hotel" : true
}
}
]
}
Destinations :
{
"_id" : ObjectId("5ec5ae9037ea99f20a79071a"),
"name" : "dest 1",
"country" : "country name"
}
{
"_id" : ObjectId("5ec5ae8e37ea99f20a78ef8c"),
"name" : "dest 2",
"country" : "country name"
}
{
"_id" : ObjectId("5ec5ae8e37ea99f20a78ef8d"),
"name" : "dest 3",
"country" : "country name"
}
{
"_id" : ObjectId("5ecf994435c3a6025d5bf126"),
"name" : "dest 4",
"country" : "country name"
}
Expected result :
{
"_id" : ObjectId("5f3122f4d8d57e3b9650e5b4"),
"title" : "tour 1",
"locations" : [
{
"destination" : {
"id" : "5ec5ae9037ea99f20a79071a",
"name" : "dest 1",
"country" : "country name"
},
"services" : {
"hotel" : true
}
},
{
"destination" : {
"id" : "5ec5ae8e37ea99f20a78ef8c",
"name" : "dest 2",
"country" : "country name"
},
"services" : {
"hotel" : true
}
}
]
},
{
"_id" : ObjectId("5f2d65e68bc6e9155310d147"),
"title" : "tour 2",
"locations" : [
{
"destination" : {
"id" : "5ecf994435c3a6025d5bf126",
"name" : "dest 4",
"country" : "country name"
},
"services" : {
"hotel" : true
}
}
]
},
{
"_id" : ObjectId("5f2d66398bc6e9155310d161"),
"title" : "tour 3",
"locations" : [
{
"destination" : {
"id" : "5ec5ae8e37ea99f20a78ef8d",
"name" : "dest 3",
"country" : "country name"
},
"services" : {
"hotel" : true
}
}
]
}
Tried query :
db.tours.aggregate([
{
"$addFields": {
"locations": {
"$map": {
"input": "$locations",
"in": {
"$mergeObjects": [
"$$this",
{
"dest_oid": {
"$toObjectId": "$$this.destination.id"
}
}
]
}
}
}
}
},
{ "$unwind": "$locations" },
{ "$lookup": {
"from": "destinations",
"localField": "locations.dest_oid",
"foreignField": "_id",
"as": "locations.dest",
}},
{ "$unwind": "$locations.dest" },
{ "$group": {
"_id": "$_id",
"locations": { "$push": "$locations" }
}}
])
even i have tried this
MongoDB $lookup on nested document

Answer 1

Quick fixes,
$unwind locations array put first because need to convert id to object id
db.tours.aggregate([
{ $unwind: "$locations" },
you skip this part if you have already converted string id t object id
$addFields replace locations.destination.id to object id filtered your logic to short, here no need $map and $mergeObjects options
{
$addFields: {
"locations.destination.id": {
$toObjectId: "$locations.destination.id"
}
}
},
$lookup that you have already did, but change as locations.destination
{
$lookup: {
from: "destinations",
as: "locations.destination",
localField: "locations.destination.id",
foreignField: "_id"
}
},
$unwind locations.destination because its array and we need object
{
$unwind: {
path: "$locations.destination"
}
},
$group that you have already did, few changes, push first destination and services in locations and add first title
{
$group: {
_id: "$_id",
locations: { $push: "$locations" },
title: { $first: "$title" }
}
}
])
Playground: https://mongoplayground.net/p/yaTCij7NRUj

Answer 2

If you can convert/update the string destination.id in the Tours collection to ObjectId if they are not already. Following query should work.
Query:
db.Tours.aggregate([
{
$unwind: "$locations",
},
{
$lookup: {
from: "Destinations",
localField: "locations.destination.id",
foreignField: "_id",
as: "destination",
},
},
{
$project: {
title: "$title",
locations: {
destination: {
$arrayElemAt: ["$destination", 0],
},
services: "$locations.services",
},
},
},
{
$group: {
_id: "$_id",
title: {
$first: "$title",
},
locations: {
$push: "$locations",
},
},
},
]);
Playground Link