Я сделал так
<%= form_for @festival do |festival_form| %>
<div id="social_networks_fields">
<%= festival_form.fields_for :social_networks do |social_network_fields| %>
<%= render 'social_network_fields', f: social_network_fields %>
<% end %>
</div>
<% end %>
<!--_social_network_fields.html.erb-->
<%= f.select :network, SocialNetwork.networks.keys %>
<%= f.text_field :link %>
У модели SocialNetwork долно быть поле network как enum.
class SocialNetwork < ActiveRecord::Base
belongs_to :festivals
enum network: [:facebook, :vkontakte, :twitter, :linkedin, :vimeo, :flickr,
:google, :youtube]
end
