_errors.haml

- if object.errors.any?
  .alert.alert-danger.fade.in
    %a.close{"data-dismiss" => 'alert'} ×
    %ul
      - object.errors.each do |attr, msg|
        - if msg.first == "^"
          = content_tag :li, msg[1..-1] if msg.is_a? String
        - else
          = content_tag :li, "#{object.class.human_attribute_name(attr)} #{msg}" if msg.is_a? String

course_registration.rb

class CourseRegistration < ActiveRecord::Base
  belongs_to :course
  belongs_to :user

  validates :course, :user, :first_name, :last_name, :age, presence: true
  validates :last_name,
    uniqueness: {
      scope: [:first_name, :course_id, :user_id],
      case_sensitive: false,
      message: "^This student has already been registered."
    }
  validates :age,
    numericality: {
      only_integer: true, greater_than: 5
    }

  def name
    "#{first_name} #{last_name}"
  end
end