app/views/tasks/_add_prev_relation.html.erb
<%= form_tag('/prev_task_relationships',:method => :post, remote: true) do %>
<% if task.candidate_task.present? %>
<div><%= hidden_field_tag :next_task, task.id %></div>
<%= collection_select :prev_task, :id, task.candidate_task, :id, :name %>
<%= submit_tag t("default.add"),class: "btn btn-xs btn-primary" %>
<% end %>
<% end %>