How can I write an RSpec test for link_to_add_fields helper from Railscast 197

Question

I used the approach details in Railscast espisode #197 on how to add dynamic input fields, but I'm not sure how to write an RSpec test for it. Suggestions?

def link_to_add_fields(name, f, association, path, *args)
  new_object = f.object.class.reflect_on_association(association).klass.new
  fields = f.semantic_fields_for(association, new_object, :child_index => "new_#{association}") do |builder|
    render(path + association.to_s.singularize + "_fields", :builder => builder)
  end
  link_to_function(name, "add_fields(this, \"#{association}\", \"#{escape_javascript(fields.html_safe)}\")", *args)
end