How do I 'validate' on destroy in rails

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  endend

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.

just a note:

For rails 3

class Booking < ActiveRecord::Basebefore_destroy :booking_with_payments?privatedef booking_with_payments?        errors.add(:base, "Cannot delete booking with payments") unless booking_payments.count == 0        errors.blank? #return false, to not destroy the element, otherwise, it will delete.end

It is what I did with Rails 5:

before_destroy do  cannot_delete_with_qrcodes  throw(:abort) if errors.present?enddef cannot_delete_with_qrcodes  errors.add(:base, 'Cannot delete shop with qrcodes') if qrcodes.any?end