app/views/admin/designations/_form.html.erb
<%= form_for resource_instance_name, :url => collection_path, :remote => true do |f| %>
<%= error_messages_for(@designation) %>
<label>Taxonomy</label>
<%= f.select :taxonomy_id,
options_from_collection_for_select(@taxonomies, :id, :name, @designation && @designation.taxonomy_id)
%><br />
<label>Name</label><%= f.text_field :name %><br />
<% end %>