validate :image_remote_url_change
  def image_remote_url_change
    if self.image_remote_url.blank? and self.image_remote_url_changed?
      errors.add(:field, "can't be blank once set")
      return false
    end
  end

validate :custom_validation_function

def custom_validation_function
  if self.field.blank? and self.field_changed?
      errors.add(:field, "can't be blank once set")
      return false
  end
end