Firstly, you don't necessarily need to follow RDB relation concepts on Mongo DB. Mongo provides flexibility for keeping redudant data with very negligible overhead to make retrieval operations fast. So you can keep User ID inside Article model although you have array of Article ID references inside User model.
Secondly, I'd not recommend you to keep reference of Article doc inside User as all articles will load with every deafult find query you make on User model. Instead, store Article IDs as raw ObjectIds inside array and whenever you want to operate on articles inside User query then you can use $lookup operator inside aggregation or simply do a find a query on Article with matching User ID because anyway Document IDs are indexed by deafult so performance wont hamper much.
I hope I've answered your question. Cheers!