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 database has CUSTOMERS and each customer can have multiple ORDERS, I want to prevent someone from deleting a customer if it has any orders in the database. They should only be able to delete a customer if it has no orders.

Is there a way when defining the association between models to enforce this behavior?

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