Initial situation

_form.html.erb

<%= form_for(@user) do |f| %>

    <fieldset>
    <div class="clearfix <%= @user.errors[:login].present? ? :error : nil %>">
      <%= f.label :login %><br />
      <div class="input">
        <span class="help-block">
          Your login. You know... login.
        </span>

        <%= f.text_field :login, :class => "xxlarge" %>

        <% @user.errors[:login].each do |err| %>
          <span class="help-block">
            <p><%= err %></p>
          </end>
        <% end%>
      </div>
    </div>
 </fieldset>

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

user.rb

class User < ActiveRecord::Base
  validates :login, :length => { :in => 2..10 }
end