Conditional callbacks

- [Voiceover] In this movie,…we'll learn about conditional callbacks,…or callbacks which run only under certain circumstances.…In order to make a conditional callback,…what we need to do is add those conditions…to the callback registration methods.…And we do that by adding options to that registration.…We use :if and :unless options.…So, for example, let's say we had an after_save callback,…whose method it called was :rejoice,…then we could put a comma, and then as an option…we could use either :if or :unless…and then the value that goes with that option…is going to be either a method or a code block.…

And that method or code block…we would expect to return true or false,…because that's what we're doing,…we're evaluating a Boolean.…Should this callback be run or not?…Well it will only be run…if that method or block returns true.…Or, in the case of :unless, it will only run…if that method or block returns false.…So let's look at an example.…In the last movie, we registered…a before_validation callback on our category object.…