Use fields that start with $ in MongoDB aggregation lookup - mongodb

Question

I have a MongoDB database that is populated by a Spring application using Spring Data. I want to perform a manual query to join two collections and extract some statistics from this data.
The first collection is named emailCampaign and contains this information (simplified):
{
"_id" : ObjectId("5db85687307b0a0d184448db"),
"name" : "Welcome email",
"subject" : "¡Welcome {{ user.name }}!",
"status" : "Sent",
"_class" : "com.mycompany.EmailCampaign"
}
The second collection is named campaignDelivery and contains this information (simplified):
/* 1 */
{
"_id" : ObjectId("5db183fb307b0aef3113361f"),
"campaign" : {
"$ref" : "emailCampaign",
"$id" : ObjectId("5db85687307b0a0d184448db")
},
"deliveries" : 3,
"_class" : "com.mycompany.CampaignDelivery"
}
/* 2 */
{
"_id" : ObjectId("5db85f2c307b0a0d184448e1"),
"campaign" : {
"$ref" : "emailCampaign",
"$id" : ObjectId("5db85687307b0a0d184448db")
},
"deliveries" : 5,
"_class" : "com.mycompany.CampaignDelivery"
}
Ultimately I want to obtain the sum of both deliveries field, but by now I'm stuck with the basic JOIN:
db.emailCampaign.aggregate([
{
$lookup: {
from: 'campaignDelivery',
localField: '_id',
foreignField: 'campaign.$id',
as: 'deliveries'
}
}
])
Throws the following error:
FieldPath field names may not start with '$'.
Escaping the dollar had no impact whatsoever, and I can't any examples of fields that start with dollars.

Answer 1

You can workaround it by using uncorrelated $lookup with $objectToArray in the sub-query to access campaign.$id:
db.emailCampaign.aggregate([
{ $lookup: {
from: "campaignDelivery",
let: { id: "$_id" },
pipeline: [
{ $addFields: {
refId: { $arrayElemAt: [
{ $filter: {
input: { $objectToArray: "$campaign" },
cond: { $eq: [ "$$this.k", { $literal: "$id" } ] }
} }
, 0
] }
} },
{ $match: {
$expr: { $eq: [
"$refId.v",
"$$id"
] }
} },
{ $project: {
refId: 0
} }
],
as: "deliveries"
} }
])