I have a Rails application with Twitter Bootstrap that, when creating a record and displaying errors, shows the error in a flash / notice as the image: 
However, I would like errors to appear in the fields, and change the input class to .has-error or something. I took a look at the field_error_proc and tried to change it in several ways, however, without success (actually, maybe I did something wrong).
Using form helpers
<%= form_for @post |f| %>
<% @post.errors[:title].join(", ") %>
<%= f.text_field :title %>
<%= f.submit "Create" %>
<% end %>
Error fields have an output like this:
<div class="field_with_errors">
<input id="post_title" name="post[title]" size="30" type="text" value="">
</div>
using class field_with_errors :
.field_with_errors {
padding: 2px;
background-color: red;
display: table;
}
In the case of Bootstrap, you can make the border of fields with red error with a simple line in jQuery:
$(".form-group:has(.field_with_errors)").addClass("has-error");