_form.html.erb

<%= form_for(setup_user @user) do |f| %>
  <% if @user.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@user.errors.count, "error") %> prohibited this user from being saved:</h2>

      <ul>
      <% @user.errors.full_messages.each do |msg| %>
        <li><%= msg %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div>
    <%= f.label :email %>
    <%= f.text_field :email %>
  </div>

  <%= f.fields_for :address do |ff| %>
    <div>
    <%= ff.label :city %>
    <%= ff.text_field :city %>
    </div>
  <% end %>

  <%= f.fields_for :interest_users do |ff| %>
    <div>
    <%= ff.hidden_field :interest_id %>
    <%= ff.check_box :_destroy, {:checked => ff.object.persisted?}, '0', '1' %>
    <%= ff.label :_destroy, ff.object.interest.name %>
    </div>
  <% end %>

  <h2>Tasks</h2>
  <%= f.fields_for :tasks do |ff| %>
    <div>
      <%= ff.label :name %>
      <%= ff.text_field :name %>

      <% if ff.object.persisted? %>
        <%= ff.check_box :_destroy %>
        <%= ff.label :_destroy, "Destroy" %>
      <% end %>
    </div>

  <% end %>

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