## The model [ruby]

class User
  attr_accessor :username
end

## Controller code [ruby]

@user = User.new
@user.username = "kalle"

## View [html_rails]

<div id="form">
  <% form_tag(:for => @user, :method => :post) do |form| %>
    <%= form.text :username, :label => 'My username:' %>
    <%= form.hidden :creditcard %>
    <%= form.submit "Save this shit" %>
  <% end %>
</div>

## Output [html]

<div id="form">
  <form method="post">
    <p>
      <label for='user_username'>My username:</label>
      <input id='user_username' name='user[username]' type='text' value='kalle' />
    </p>
    <p>
      <label for='user_age'>Age:</label>
      <input id='user_age' name='user[age]' type='text' value='' />
    </p>
    <p>
      <input id='user_extra' name='user[extra]' type='text' value='' />
    </p>
  </form>
</div>