app/views/studies/_add_related_study.html.erb
<div style="text-align:center">
<%= form_for :related_study, url: relate_study_study_path(id: @study) do -%>
<label for="related_study_study_id">Select a study</label> <%= select "related_study", "study_id", @studies.map {|r| [r.name, r.id]}, { prompt: true } %>
<label for="related_study_relation_type">then assign a study relation </label> <%= select "related_study", "relation_type", @relation_names, { prompt: true, } %>
<%= submit_tag "Add related study" %>
<% end -%>
</div>