I recently had to combine both polymorphic and has_and_belongs_to_many relationship between different models in our Mongo DB. The challenge is that when I store relationship IDs in array with the record, I cannot define the relationship_type. Solution is to go with mapping table and define polymorphic relationship there.
You can do article.authors, user.articles and organization.articles. In retrospect the solution was fairly straightforward but I could not find any examples online so I decided to write my own blog post.