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 1

Use ActiveRecord context validation in Rails 5.

class ApplicationRecord < ActiveRecord::Base
  before_destroy do
    throw :abort if invalid?(:destroy)
  end
end

class Ticket < ApplicationRecord
  validate :validate_expires_on, on: :destroy

  def validate_expires_on
    errors.add :expires_on if expires_on > Time.now
  end
end

Answer 2

just a note:

For rails 3

class Booking < ActiveRecord::Base

before_destroy :booking_with_payments?

private

def 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

Answer 3

You can wrap the destroy action in an "if" statement in the controller:

def destroy # in controller context
  if (model.valid_destroy?)
    model.destroy # if in model context, use `super`
  end
end

Where valid_destroy? is a method on your model class that returns true if the conditions for destroying a record are met.

Having a method like this will also let you prevent the display of the delete option to the user - which will improve the user experience as the user won't be able to perform an illegal operation.

Answer 4

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.

Answer 5

The ActiveRecord associations has_many and has_one allows for a dependent option that will make sure related table rows are deleted on delete, but this is usually to keep your database clean rather than preventing it from being invalid.

Answer 6

I ended up using code from here to create a can_destroy override on activerecord: https://gist.github.com/andhapp/1761098

class ActiveRecord::Base
  def can_destroy?
    self.class.reflect_on_all_associations.all? do |assoc|
      assoc.options[:dependent] != :restrict || (assoc.macro == :has_one && self.send(assoc.name).nil?) || (assoc.macro == :has_many && self.send(assoc.name).empty?)
    end
  end
end

This has the added benefit of making it trivial to hide/show a delete button on the ui