How to raise a validation error in Rails after save

gistfile1.rb

# This probably isn't a good thing to want to do, but it came up for me,
# so in the spirit of helping others with weird problems (and because this
# seems to be documented almost nowhere):

after_save do
  
  if some_failing_condition
    errors.add(:something, "some failure happened.")
    raise ActiveRecord::RecordInvalid.new(self)
  end
  
end