如何将 Ruby class 实例变量初始化为另一个 class 的新实例？

Question

I am working on a Ruby on Rails 6 project, and I am trying to use a class instance variable on an ActiveRecord model.Here is a basic example:

class Model << ApplicationRecord
  @var = AnotherClass.new
  
  class << self
    attr_reader :var
  end
  
  # ...
end

I would then like to be able to use Model.var to access Model 's instance of AnotherClass . There are multiple such models, each of them referring to a different AnotherClass , with all the AnotherClass es being subclasses of some BaseClass .

However, I am encountering the following error:

uninitialized constant Model::AnotherClass

Because of the class << self , Ruby seems to be looking for a nested class.

Is there a way to access AnotherClass directly, or is there a better way in general to set this up?

Edit : I solved this with a completely different approach, however I'm still interested to see how you would get around this issue.

Answer 1

The error you receive:

uninitialized constant Model::AnotherClass

Tells you that AnotherClass is not initialized (not loaded/found). Let me use the following context as an example:

class Model
  AnotherClass
end

Ruby will start a constant lookup. This will start from the current namespace ( Model ) and and if nothing is found move up into the namespace tree. In the above example it will first look for Model::AnotherClass if that cannot be found it will look for AnotherClass , if that cannot be found it will throw the exception you receive.

This error simply tells you that AnotherClass is not loaded.

Anything in th app/ directory is loaded by the autoloader of Rails, however if you use the lib/ directory you have to manually require 'another_class' or add the relevant path to the autoload paths .