How do I copy the primary keys of documents from one collection and save it in another collection in a similar database

Working with Data
1

I want to use $lookup to join two collections in my database. I need to take primary keys("_id") from one collection and add them to another collection as “id2” in order to use lookup. How can I perform this operation?

2

1

I want to copy the “_id” for this collection to another collection and save it as “_id2”

in another collection. And them perform $lookup and get both the collections together with my foreignField being “_id2”.

3

Untested:

collection_one = ...
collection_two = ...

document_one = { ... }
collection_one.insertOne( document_one ) 
document_two = { "_id2" : document_one._id }
collection_two.insertOne( document_two )

lookup = { "$lookup" : {
  "from" : "collection_two" ,
  "localField" : "_id" ,
  "foreignField" : "_id2" ,
  "as" : "two" 
} } 

collection_one.aggregate( lookup )