1. show.html.erb

<!-- events/show.html.erb -->

<%= form_tag(event_medfiles_release_many_path, method: 'POST') do %>
  <%= hidden_field_tag 'id', @event.id %>

  <% @event.medfiles.each do |medfile| %>
    <%= render(partial: 'medfiles/release_form', object: medfile, as: 'medfile') %>
  <% end %>
  <%= submit_tag 'Do The Thing' %>
<%= end %>

2. _release_form.html.erb

<!-- medfiles/_release_form.html.erb -->

<div>
  Whatever text you need and also stuff from medfile <%= medfile.id %>,
  since <pre>medfile</pre> is a local variable in this context.

  Also <%= check_box_tag "publish[]" value="#{medfile.id}" %>
</div>

3. routes.rb

# config/routes.rb
# (somewhere near your other event and/or medfile resources)

  resources :events do
    post 'medfiles/release_many', to: 'medfiles#release_many'
  end