form_for example rails 4.html.erb

<%= form_for(@category) do |f| %>
  <% if f.object.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(f.object.errors.count, "error") %> prohibited this category from being saved:</h2>
      <ul>
      <% f.object.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %>
    <%= f.text_field :name %>
  </div>

  <%= f.fields_for :sub_categories do |sub_cat_form| %>
    <div class="field">
      <%= sub_cat_form.label "sub_category name" %>
      <%= sub_cat_form.text_field :name %>
    </div>
  <% end %>

  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>