Mongodb $unwind removes objects. if $lookup doesn't have reference [duplicate] - mongodb

Question

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Handling unwind for the non existing embedded document [duplicate]
(1 answer)
Closed 3 years ago.
If country doesn't have reference states and cities. $unwind removes country name from the collections.
Expected Output will be Mongodb should return country name even if the country doesn't any states and cities reference.
Country Collection:
[
{
"_id": "5d052c76df076d23a48d4a3b",
"name": "India"
},
{
"_id": "5d052c76df076d23a48d4b07",
"name": "Indonesia"
},
{
"_id": "5d052c76df076d23a48d22f4",
"name": "Iran"
}
]
State Collection:
[
{
"_id": "5d2236c37ed1112b3cc41397",
"name": "Andaman and Nicobar Islands",
"countryId": "5d052c76df076d23a48d4a3b"
},
{
"_id": "5d2236c37ed1112b3cc41398",
"name": "Andhra Pradesh",
"countryId": "5d052c76df076d23a48d4a3b"
}
]
City Collection:
[
{
"name": "Port Blair",
"stateId": "5d2236c37ed1112b3cc41397"
},
{
"name": "Adoni",
"stateId": "5d2236c37ed1112b3cc41398"
}
]
Query:
Country.aggregate([
{
$lookup:{
from: 'states',
localField:'_id',
foreignField:'countryId',
as:'states'
}
},
{
$unwind: {
path: "$states"
}
},
{
$lookup:{
from: 'cities',
localField:'states._id',
foreignField:'stateId',
as:'states.cities'
}
},
{
$group: {
_id: {
_id: '$_id',
name: '$name'
},
states: {
$push: '$states'
}
}
},
{
$project: {
_id: '$_id._id',
name: '$_id.name',
states: 1
}
}
])
Output:
[
{
"_id":"5d052c76df076d23a48d4a3b",
"name":"India",
"states":[
{
"_id":"5d2236c37ed1112b3cc41397",
"name":"Andaman and Nicobar Islands",
"countryId":"5d052c76df076d23a48d4a3b",
"cities":[
{
"name":"Port Blair",
"stateId":"5d2236c37ed1112b3cc41397"
}
]
},
{
"_id":"5d2236c37ed1112b3cc41398",
"name":"Andhra Pradesh",
"countryId":"5d052c76df076d23a48d4a3b",
"cities":[
{
"name":"Adoni",
"stateId":"5d2236c37ed1112b3cc41398"
}
]
}
]
}
]
Expected Output:
[
{
"_id":"5d052c76df076d23a48d4a3b",
"name":"India",
"states":[
{
"_id":"5d2236c37ed1112b3cc41397",
"name":"Andaman and Nicobar Islands",
"countryId":"5d052c76df076d23a48d4a3b",
"cities":[
{
"name":"Port Blair",
"stateId":"5d2236c37ed1112b3cc41397"
}
]
},
{
"_id":"5d2236c37ed1112b3cc41398",
"name":"Andhra Pradesh",
"countryId":"5d052c76df076d23a48d4a3b",
"cities":[
{
"name":"Adoni",
"stateId":"5d2236c37ed1112b3cc41398"
}
]
}
]
},
{
"_id":"5d052c76df076d23a48d4b07",
"name":"Indonesia",
"states":[
]
},
{
"_id":"5d052c76df076d23a48d22f4",
"name":"Iran",
"states":[
]
}
]

Answer 1

just add " preserveNullAndEmptyArrays: true " to $unwind
Country.aggregate([
{
$lookup:{
from: 'states',
localField:'_id',
foreignField:'countryId',
as:'states'
}
},
{
$unwind: {
path: "$states",
preserveNullAndEmptyArrays: true
}
},
{
$lookup:{
from: 'cities',
localField:'states._id',
foreignField:'stateId',
as:'states.cities'
}
},
{
$group: {
_id: {
_id: '$_id',
name: '$name'
},
states: {
$push: '$states'
}
}
},
{
$project: {
_id: '$_id._id',
name: '$_id.name',
states: 1
}
}
])
output
[
{
"states" : [
{
"cities" : []
}
],
"_id" : "5d052c76df076d23a48d22f4",
"name" : "Iran"
},
{
"states" : [
{
"cities" : []
}
],
"_id" : "5d052c76df076d23a48d4b07",
"name" : "Indonesia"
},
{
"states" : [
{
"_id" : "5d2236c37ed1112b3cc41397",
"name" : "Andaman and Nicobar Islands",
"countryId" : "5d052c76df076d23a48d4a3b",
"cities" : [
{
"_id" : ObjectId("5d38ccb6f9c5fa48bf099027"),
"name" : "Port Blair",
"stateId" : "5d2236c37ed1112b3cc41397"
}
]
},
{
"_id" : "5d2236c37ed1112b3cc41398",
"name" : "Andhra Pradesh",
"countryId" : "5d052c76df076d23a48d4a3b",
"cities" : [
{
"_id" : ObjectId("5d38ccbcf9c5fa48bf09902a"),
"name" : "Adoni",
"stateId" : "5d2236c37ed1112b3cc41398"
}
]
}
],
"_id" : "5d052c76df076d23a48d4a3b",
"name" : "India"
}
]