`badges_sashes` is a many-to-many join model for AR. Do you think it makes sense to model it the same in Mongoid?

Also, AR's definition for `add_badge` is:
```ruby
      bs = BadgesSash.new(badge_id: badge_id)
      badges_sashes << bs
      bs
```
Do you think we could define one same version for both ORMs, in `Base`?