Ruby on Rails: errors.add_to_base vs. errors.add

I have read that errors.add_to_base should be used for errors associated with the object and not a specific attribute. I am having trouble conceptualizing what this means. Could someone provide an example of when I would want to use each?

For example, I have a Band model and each Band has a Genre. When I validate the presence of a genre, if the genre is missing should the error be added to the base?

The more examples the better

Thank you!

It's worth noting (since this shows up in the search engines, which is how I found it) that this is deprecated. The Rails 3 way of doing it is below but is no longer valid as of Rails 7 (see the comment from April 2022)

errors[:base] << "Error message"    

the preferred way of doing it is

errors.add(:base, "Error message")

http://apidock.com/rails/ActiveRecord/Errors/add_to_base
http://apidock.com/rails/v3.2.8/ActiveModel/Errors/add

A missing genre would be a field error. A base error would be something like an exact duplicate of an existing record, where the problem wasn't tied to any specific field but rather to the record as a whole (or at lest to some combination of fields).

In this example, you can see field validation (team must be chosen). And you can see a class/base level validation. For example, you required at least one method of contact, a phone or an email:

class Registrant
  include MongoMapper::Document

  # Attributes ::::::::::::::::::::::::::::::::::::::::::::::::::::::
  key :name, String, :required => true
  key :email, String
  key :phone, String

  # Associations :::::::::::::::::::::::::::::::::::::::::::::::::::::
  key :team_id, ObjectId
  belongs_to :team
...
  # Validations :::::::::::::::::::::::::::::::::::::::::::::::::::::
  validate :validate_team_selection
  validate :validate_contact_method
...

  private

  def validate_contact_method
    # one or the other must be provided
    if phone.empty? and email.empty?
      errors.add_to_base("At least one form of contact must be entered: phone or email" )
    end
  end

  def validate_team_selection
    if registration_setup.require_team_at_signup
      if team_id.nil?
        errors.add(:team, "must be selected" )
      end
    end
  end
end