Rails helper to get validation error messages for a given object or an optional field on that object. Also a view partial to output those errors.

_errors.html.erb

<% if object.errors.present? %>
  <div class="alert alert-danger">
    <%
    field ||= nil
    error_messages_for(object, field) do |error|
    %>
      <%= error %>
      <br/>
    <% end %>
  </div>
<% end %>

application_helper.rb

module ApplicationHelper
  def error_messages_for(object, field=nil)
    errors = field ? object.errors[field] : object.errors.full_messages
    errors.each do |error|
      yield(error)
    end
  end
end