If you'd like to use business logic to determine when to show what partial for your @exercises
collection you should use the to_partial_path
method in the Exercise model to define that. See #4 in this post: http://blog.plataformatec.com.br/2012/01/my-five-favorite-hidden-features-in-rails-3-2/
Or, if this is more of a view-related decision (i.e. one view will always use the regular _exercises.html.erb and another view would always use e.g. _alternate_exercises.html.erb) then you can specify as such:
<%= render partial: 'alternate_exercises', collection: @exercises, as: :exercise %>
This will render the _alternate_exercises.html.erb partial once for each item in @execrises
passing the item in to the partial via a local_assign called exercise
.