How do I 'validate' on destroy in rails

Question

On destruction of a restful resource, I want to guarantee a few things before I allow a destroy operation to continue? Basically, I want the ability to stop the destroy operation if I note that doing so would place the database in a invalid state? There are no validation callbacks on a destroy operation, so how does one "validate" whether a destroy operation should be accepted?

Answer

You can raise an exception which you then catch. Rails wraps deletes in a transaction, which helps matters.

For example:

class Booking < ActiveRecord::Base
  has_many   :booking_payments
  ....
  def destroy
    raise "Cannot delete booking with payments" unless booking_payments.count == 0
    # ... ok, go ahead and destroy
    super
  end
end

Alternatively you can use the before_destroy callback. This callback is normally used to destroy dependent records, but you can throw an exception or add an error instead.

def before_destroy
  return true if booking_payments.count == 0
  errors.add :base, "Cannot delete booking with payments"
  # or errors.add_to_base in Rails 2
  false
  # Rails 5
  throw(:abort)
end

myBooking.destroy will now return false, and myBooking.errors will be populated on return.