mongodb `$lookup` or `join` with attributes inside array of objects - mongodb

Question

I have this object that coming from mongodb
[
{
"_id": "5eaf2fc88fcee1a21ea0d94d",
"migration_customer_union_id": 517,
"__v": 0,
"account": 1,
"createdAt": "2020-05-03T20:55:36.335Z",
"customerUnion": "5eaf2fc7698de8321ccd841d",
"shaufel_customers": [
{
"percent": 50,
"_id": "5eaf2fc8698de8321ccd881f",
"customer": "5eaf2fb9698de8321ccd68c0"
},
{
"percent": 50,
"_id": "5eaf2fc9698de8321ccd8a9d",
"customer": "5eaf2fb9698de8321ccd68c0"
}
],
}
]
you can notice inside shaufel_customers array there is an attribute named customer which I want to use it to join with customers document, so that's what I am doing (wrote this code with help of stackoverflow :) )
const aggregate = await CustomerUnionCustomer.aggregate(
[
{
$match: {migration_customer_union_id: 517}
},
{
$lookup: {
from: 'customers',
localField: 'shaufel_customers.customer',
foreignField: '_id',
as: 'customers',
}
},
{
$project: {
shaufel_customer_union_id: 1,
customerUnion: '$customerUnions',
shaufel_customers: {
$map: {
input: "$customers",
as: "c",
in: {
$mergeObjects: [
"$$c",
{
$arrayElemAt: [{
$filter: {
input: "$shaufel_customers",
cond: {$eq: ["$$this.customer", "$$c._id"]}
}
}, 0]
},
]
}
},
}
}
},
{
"$project": { // this project just to get some specific values inside shaufel_customers
'_id': 0,
"shaufel_customers": {
"$map": {
"input": "$shaufel_customers",
"as": "customer",
"in": {
"customer_id": "$$customer.shaufel_customer_id",
"percent": "$$customer.percent"
}
}
}
}
}
]
)
when executing this code I am getting the following response
[
{
"shaufel_customers": [
{
"customer_id": "869",
"percent": 50
}
]
}
]
you can notice I got one object, although there was two objects inside the original array above, and that's because the customer attribute above has the same ObjectId value 5eaf2fb9698de8321ccd68c0 and that's what I want to ask. I want to get the same two objects even if the ids are the same, so the result I am expecting here is
[
{
"shaufel_customers": [
{
"customer_id": "869",
"percent": 50
},
{
"customer_id": "869",
"percent": 50
},
]
}
]
How should I do that :(

Answer 1

You need to revert your $map and iterate shaufel_customers instead of customer - this will return two results:
{
$project: {
shaufel_customer_union_id: 1,
customerUnion: '$customerUnions',
shaufel_customers: {
$map: {
input: "$shaufel_customers",
as: "sc",
in: {
$mergeObjects: [
"$$c",
{
$arrayElemAt: [{
$filter: {
input: "$customers",
cond: {$eq: ["$$this._id", "$$sc.customer"]}
}
}, 0]
},
]
}
},
}
}
},