'RelatedManager' object has no attribute

When you access user.details, it accesses the backreference of the UserDetail.user foreign key. The foreign Key itself doesn't specify that a User can only have one UserDetail, so django gives you a RelatedManager, which you can filter and query just like a regular Manager. So you do the same things to it that you do to your .objects managers. You can ask for user.details.all(), user.details.filter(), user.details.get(), and so on, which will either give you a queryset, an object, or an exception,depending on the method and the results.

Try

request.user.details.get().favourites.add(article)

This assumes the UserDetail object already exists for that user, otherwise get() will raise an exception.