mongodb - $lookup with $if operator - node.js

Question

I am building a side project. where I want to use nested $lookup and $unwind if certain condition match. I created simple prototype below. some users have numbers and some don't have numbers. I want to do join if the number is not null or undefined
here is the code -
users.aggregate([
{
$lookup: "number",
localField:"user.number",
foreignField:"name",
as:"number"
}
{
$unwind: {
path: "$number",
preserveNullAndEmptyArrays: true
}
}
,
{
$lookup: "countries",
localField:"number.countrycode",
foreignField:"name",
as:"countries"
},
{
$unwind:"countries",
//preserveNullAndEmptyArrays
},
$project:{
name:1,
number:"$number.phoneNumber",
countryCode:"$countries.countryCode"
}
])
Output:
[{
name:"yashraj basan"
number:"123345677",
countryCode:"US"
},
{
name:"krutik basan"
number:"123345679",
countryCode:"FR"
}]
Right now i am getting all users who have numbers but i am want both users who have number and who don't have number
Expected output:
[{
name:"yashraj basan"
number:"123345677",
countryCode:"US"
},
{
name:"krutik basan"
number:"123345679",
countryCode:"FR"
},
{
name:"dhruvam basan",
number:"",
countrycode:""
},{
name:"foo bar",
number:"",
countrycode:""
}]
I appreciate all your inputs. thank you

Answer 1

There's no need to run $lookup conditionally. You can do it for every user but then when you use $unwind you can take advantage of preserveNullAndEmptyArrays option:
{
$lookup: {
from:"countries",
localField:"user.number.countrycode",
foreignField:"name",
as:"countries"
}
},
{
$unwind: {
path: "countries",
preserveNullAndEmptyArrays: true
}
}