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Document .descendants and .subclasses advice #356

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20 changes: 20 additions & 0 deletions STYLEGUIDE.md
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Expand Up @@ -1096,4 +1096,24 @@ If you need to call a subview that expects an instance variable be set. If possi

Unfortunately the only way to get data into a layout template is with instance variables. You can't explicitly pass locals to them.


### Avoid `Class#descendants` and `Class#subclasses`

Avoid using `Class#descendants` and `Class#subclasses` as they are unreliable for several reasons:

* They don't know about classes that haven't been autoloaded yet
* They're non-deterministic with regards to garbage collection of classes. If you use `Class#descendants` or `Class#subclasses` in tests, where there is a pattern to dynamically define classes, GC is unpredictable for when those classes are cleaned up and removed by the GC.

```ruby
# bad
class Person < ApplicationRecord
end

class Employee < Person
end

Person.descendants # => Unreliable, may or may not include Employee
Person.subclasses # => Unreliable, may or may not include Employee
```

[rubocop-guide]: https://github.com/rubocop-hq/ruby-style-guide