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

Question

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!

Answer 1

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:

errors[:base] << "Error message"     

or

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

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

Answer 2

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).