How to find data with condition on a second collection with Mongoose - node.js

Question

I have a collection "campanas" with similar structure to:
{
_id: ObjectId,
name: String,
starts: Date,
ends: Date,
available: Boolean,
max_requirements: Int,
}
Whenever a user interacts with the Campana a requirement is generated with this structure in a different collection called "campana_requrements":
{
_id: ObjectId,
id_campana: {type: ObjectId, ref: "campanas"},
id_user: {type: ObjectId, ref: "users",
action_needed: String
}
Campanas also has attributes that are stored in its own collection called "campanas_attributes"
{
_id: ObjectId,
attribute_name: String
}
and campana and attributes are related through a 3rd collection called "campana_campana_attribute"
{
_id: ObjectId,
id_campana: {type: ObjectId, ref: "campanas"},
id_campana_attribute: {type: ObjectId, ref:"campana_attributes"}
}
So campanas I need to retrieve Campanas with its attributes BUT filter out the ones which has requirements generated by the user, in pseudo code the condition would be
campanas.max_requirements < requirements.length WHERE requirements.id_campana = campana._id AND requirements.id_user = $user
where $user is a variable I would provide.
I can easily find the campanas and add its attributes, but i cant find a way to filter depending on amount of requirements using only mongodb / mongoose methods.
I want to achieve this in the query, not with nodejs.
How can i achived this?
UPDATE
I almost got it working using Studio3T aggregation tool but the reference of "$max_requirements" fails on the $match statement, its not taking the value. It works like a charm if i hard code a 1 into it but it might not be a 1 in the future.
Heres the aggregation:
// Requires official MongoShell 3.6+
campanas.aggregate(
[
{
"$lookup" : {
"from" : "tbl_campana_requirements",
"localField" : "_id",
"foreignField" : "id_campana",
"as" : "requirements",
"pipeline" : [
{
"$match" : {
"$or" : [
{
"id_user" : ObjectId("someId")
},
{
"id_user" : null
}
]
}
}
]
}
},
{
"$addFields" : {
"req_count" : {
"$size" : "$requirements"
}
}
},
{
"$match" : {
"habilitado" : true,
"$or" : [
{
"req_count" : {
"$lt" : "$max_requirements" <--this is the problem
}
},
{
"max_requirements" : {
"$eq" : 0.0
}
}
]
}
},
{
"$lookup" : {
"from" : "tbl_campana_campana_atributos",
"localField" : "_id",
"foreignField" : "id_campana",
"pipeline" : [
{
"$lookup" : {
"from" : "tbl_campana_atributos",
"localField" : "id_campana_atributo",
"foreignField" : "_id",
"as" : "atributo"
}
},
{
"$project" : {
"_id" : 0.0,
"atributo" : "$atributo"
}
},
{
"$unwind" : "$atributo"
}
],
"as" : "atributos"
}
},
{
"$project" : {
"atributos" : "$atributos.atributo",
"habilitado" : 1.0,
"nombre" : 1.0,
"titulo" : 1.0,
"descripcion" : 1.0,
"fecha_inicio" : 1.0,
"fecha_fin" : 1.0,
"id_campana_responsable" : 1.0,
"url_imagen_banner" : 1.0,
"url_imagen_fondo" : 1.0,
"url_imagen_principal" : 1.0,
"condiciones_terminos" : 1.0
}
}
],
{
"allowDiskUse" : false
}
);