Styling form error message - bootstrap/rails

Take a look at how Michael Hartl does it in railstutorial.

And thats the used css:

#error_explanation {
  color: #f00;
  ul {
    list-style: none;
    margin: 0 0 18px 0;
  }
}

.field_with_errors {
  @extend .control-group;
  @extend .error;
 }

He describes everything here.

If you also want the little star at the beginning of every line you have to include it in your form:

     <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>    <--- insert here
          <% end %>
        </ul>
     </div>
      ...

A little late I realize, but I just ran into this today with Rails 4 and Bootstrap 3, I ended up making a view helper to display errors using a panel:

Rails 4 / Bootstrap 3

def errors_for(object)
    if object.errors.any?
        content_tag(:div, class: "panel panel-danger") do
            concat(content_tag(:div, class: "panel-heading") do
                concat(content_tag(:h4, class: "panel-title") do
                    concat "#{pluralize(object.errors.count, "error")} prohibited this #{object.class.name.downcase} from being saved:"
                end)
            end)
            concat(content_tag(:div, class: "panel-body") do
                concat(content_tag(:ul) do
                    object.errors.full_messages.each do |msg|
                        concat content_tag(:li, msg)
                    end
                end)
            end)
        end
    end
end

Rails 4 / Bootstrap 4 Beta

def errors_for(object)
    if object.errors.any?
        content_tag(:div, class: "card border-danger") do
            concat(content_tag(:div, class: "card-header bg-danger text-white") do
                concat "#{pluralize(object.errors.count, "error")} prohibited this #{object.class.name.downcase} from being saved:"
            end)
            concat(content_tag(:div, class: "card-body") do
                concat(content_tag(:ul, class: 'mb-0') do
                    object.errors.full_messages.each do |msg|
                        concat content_tag(:li, msg)
                    end
                end)
            end)
        end
    end
end

Rails 4 / Bootstrap 4 Beta List Group Variation

def errors_for(object)
    if object.errors.any?
        content_tag(:div, class: "card border-danger") do
            concat(content_tag(:div, class: "card-header bg-danger text-white") do
                concat "#{pluralize(object.errors.count, "error")} prohibited this #{object.class.name.downcase} from being saved:"
            end)
            concat(content_tag(:ul, class: 'mb-0 list-group list-group-flush') do
                object.errors.full_messages.each do |msg|
                    concat content_tag(:li, msg, class: 'list-group-item')
                end
            end)
        end
    end
end

I dropped it in application_helper and call it in my form views

<%= errors_for(@user) %>

Maybe someone will stumble upon this and find it useful.

Just in case someone stumbles here and is using Bootstrap 4 alpha with rails 5 and bootstrap_form_for gem. I use:

<div class="form-group">
  <%= f.alert_message "Please fix the errors below." %>
</div>

which looks really nice.