gistfile1.rb

class Registration < ActiveRecord::Base
  # Database Relationships
  belongs_to :person
  belongs_to :event
  
  # Validations
  validates_presence_of :event_id
  validates_presence_of :person_id
  validates_uniqueness_of :person_id, :scope => :event_id, :message => "is already signed in or registered."
  
  validate do |registration|
    registration.errors.add(:event, :passed) if registration.event && registration.event.finish < Time.now
  end
end