Use fields that start with $ in MongoDB aggregation lookup

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"
  } }
])