pipeline function in MongoDB, how to make the join - mongodb

Question

I am looking at a query in MongoDB.
Essentially, I want to join records, but only when the records in collection mongo2 meet certain conditions (those in the and statement).
I have 2 questions about this
Where can I put the local and foreign field setting. It says I cannot define them when using pipeline.
Its says that my GT and LT statements are wrong. They work in single find statements, but I am getting the error
Expression $gt takes exactly 2 arguments. 1 were passed in.
Any help will be massivel appreciated :)
Thanks guys
db.mongo.aggregate([
{ $lookup:
{
from: "mongo2",
pipeline: [
{ $match:
{ $expr:
{
$and:[{Age : {$gt:50}}, {Age : {$lt:100}}]
}
}
}
],
as: "filters"
}
}
])

Answer 1

The only way to access fields coming from mongo collection inside of pipeline is to define them as variables using let statement. For instance:
db.mongo.aggregate([
{
$lookup: {
from: "mongo2",
let: { "mongo_collection_id": "$_id" },
pipeline: [
{
$match: { $expr: { $eq: [ "$$mongo_collection_id", "$_id" ] } }
}
],
as: "filters"
}
}
])
Please note that you need double dollar sign ($$) to refer to that variable within pipeline. Single dollar references fields from mongo2 collection documents.
Answering second question: there are two $gt and $lt pairs in MongoDB (which might be confusing). Since you probably have to use $expr the only way is to use $gt (aggregation) so the syntax is a bit different:
{ $expr:
{
$and:[{ $gt: [ "$Age", 50 ] }, { $lt: [ "$Age", 100 ] }]
}
}