semiformal

controller.rb

class UsersController < ApplicationController
  def edit
    assign_user_form
  end

  def update
    if assign_user_form.commit
      redirect_to @user, :flash => "User saved."
    else
      render 'new'
    end
  end

  private

  def assign_user_form
    @user = User.find(params[:id])
    @user_form = form_for(@user) do |form|
      form.accept :email
      form.accept :password
      form.accept :password_confirmation
    end
  end
end

view.html.erb

<!-- generate all fields/buttons -->
<%= render_form @user_form %>

<!-- choose fields -->
<%= render_form @user_form do |form| -%>
  <%= form.fieldset do -%>
    <!-- custom field -->
    <%= form.field :email do |field| -%>
      <%= field.label %>
      <%= field.input %>
      <a href="#" id="check_email">Check availability</a>
    <% end -%>

    <%= form.field :password -%>
    <%= form.field :password_confirmation -%>
  <% end -%>

  <!-- default buttons -->
  <%= form.buttons -%>

  <!-- button with cancel -->
  <%= form.buttons :cancel => user_path(@user) %>

  <!-- custom buttons -->
  <%= form.buttons do -%>
    <%= form.commit_button 'Save User' %>
  <% end -%>
<% end -%>