gistfile1.rb

class Adoption < ActiveRecord::Base
  has_one :adopter_info, :as => :contact, :class_name => "ContactInfo"
  accepts_nested_attributes_for :adopter_info
end

class ContactInfo < ActiveRecord::Base

  attr_accessible :address, :email, :full_name, :phone, :state, :city, :zip
  belongs_to :contact, polymorphic: true
  validate :must_have_form_of_contact

private
  # Either phone, email, or address, but not all are required.
  def must_have_form_of_contact
    if phone.blank? && email.blank? && (address.blank? || city.blank? || state.blank? || zip.blank?)      
      errors[:base] << "You must enter a phone, address, or email"
    end
  end
end