embeds_many_to_has_many.rb

def embeds_many_to_has_many(parent, child)
  child_key_name = child.to_s.underscore.pluralize
  parent.collection.find({}).each do |parent_document|
    next unless parent_document[child_key_name]
    parent_document[child_key_name].each do |child_document|
      new_child = child_document.merge(
        "#{parent.to_s.underscore}_id": parent_document['_id']
      )
      child.collection.insert_one new_child
    end
  end
  parent.all.unset(child_key_name.to_sym)
end

embeds_many_to_has_many(Post, Comment)