How Can I Access a Class Constant from an Included Module in Rails?

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I'm working on a Rails app where the User model defines an OPTIONS constant, and I'm including a module that tries to access this constant. However, I'm getting a NameError when the module's included block executes. I've tried using self, base_class, and even referencing User directly, but I still receive the same error. Here's the setup:

RUBY
class User < ApplicationRecord
  include ModOne

  OPTIONS = {a: 1}.freeze

  ...
end

RUBY
module ModOne
  extend ActiveSupport::Concern

  included do
    do_semething(self::OPTIONS)
  end

  class_methods do
    def do_something(opts)
      ...
    end
  end
end

The error message is:

NameError: uninitialized constant User (call 'User.connection' to establish a connection)::OPTIONS
Did you mean? User::OPTIONS

What am I missing here?

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The problem is that the module’s included block is executed immediately when the module is included, and at that point the constant hasn't been defined yet in the User class. In your code, you include the module before the OPTIONS constant is defined, so when the module code tries to access self::OPTIONS, it raises a NameError.

To fix this issue, you have a couple of options:

Define the constant before including the module:
Move the OPTIONS declaration above the include statement so that it is available when the module’s included block is executed.
```
RUBY
class User < ApplicationRecord
  OPTIONS = { a: 1 }.freeze
  include ModOne

  # ...
end
```
Defer the access to the constant until after the class is fully defined:
If moving the constant is not an option, you can change the design so that the constant is accessed only after the class has been fully built (for example, via a method call or a callback).

For most cases, redefining the file order as in option 1 is the simplest solution.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by MartianExplorer654 • 1 month ago

It's simply a matter of order. You have to define the constant first. Ruby classes are really just a block of code and run top down and when you call include you're also calling the #included method on the module.

But a better approach if you want the functionality provided by a module to be customizable is to just write a so called macro method instead of hinging everything on the Module#included hook:

RUBY
# You can obscure this with some syntactic sugar from ActiveSupport::Concern 
# if it makes you happy.
module Awesomeizer
  def foo
    self.class.awesomeize_options[:foo]
  end

  module ClassMethods
    def awesomeize_options
      @awesomeize_options ||= defaults
    end

    def awesomeize(**options) 
       awesomeize_options.merge!(options)
    end 

    def defaults
      {}
    end
  end

  def self.included(base)
    base.extend(ClassMethods)
  end
end

class Thing
  include Awesomeizer
  awesomeize(foo: :bar)
end

This pattern can be found everywhere in Ruby and is great way to get around the fact that Module#include doesn't allow you to pass any additional arguments.

In this example the awesomeize method just stores the options as a class instance variable.

One of the strongest reasons why this is preferable is that it lets you define a signature for the interface between the module and it's consumers instead of just making assumptions. Some gems like Devise even use this method to include it's submodules.

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