How do I prevent deletion of parent if it has child records?

Question

I have looked through the Ruby on Rails guides and I can\'t seem to figure out how to prevent someone from deleting a Parent record if it has Children. For example. If my

Answer 1

class Customer < ActiveRecord::Base
  has_many :orders, :dependent => :restrict # raises ActiveRecord::DeleteRestrictionError

Edit: as of Rails 4.1, :restrict is not a valid option, and instead you should use either :restrict_with_error or :restrict_with_exception

Eg.:

class Customer < ActiveRecord::Base
  has_many :orders, :dependent => :restrict_with_error