Including a Validator class from a Concern in Rails

I've got a custom EachValidator that is used in two different models. I moved it to a Concern to DRY the models:

module Isbn
  extend ActiveSupport::Concern

  included do
    class IsbnValidator < ActiveModel::EachValidator
      GOOD_ISBN = /^97[89]/.freeze

      def validate_each(record, attribute, value)
       # snip...
      end
    end
  end
end

class Book < ApplicationRecord
  include Isbn

  validates :isbn, allow_nil: true, isbn: true
end

class BookPart < ApplicationRecord
  include Isbn

  validates :isbn, allow_nil: true, isbn: true
end

When running Rails (in this case via RSpec), I get this warning:

$ bundle exec rspec
C:/Users/USER/api/app/models/concerns/isbn.rb:16: warning: already initialized constant Isbn::IsbnValidator::GOOD_ISBN
C:/Users/USER/api/app/models/concerns/isbn.rb:16: warning: previous definition of GOOD_ISBN was here

Is there any way to avoid it and include the validator cleanly in each model?

Each time you include your Isbn module it triggers included method which opens IsbnValidator < ActiveModel::EachValidator class and creates GOOD_ISBN constant and validate_each method inside of it. Note that these constant and method are created each time in the same class - IsbnValidator < ActiveModel::EachValidator.

So, the first time you included Isbn module you created GOOD_ISBN constant inside IsbnValidator < ActiveModel::EachValidator, after that you included Isbn module into another class and included method tried to create GOOD_ISBN constant again in IsbnValidator < ActiveModel::EachValidator and obviously failed with that error you got.

So instead your included method should look like this:

module Isbn
  extend ActiveSupport::Concern

  included do
    GOOD_ISBN = /^97[89]/.freeze

    def validate_each(record, attribute, value)
     # snip...
    end
  end
end

This way GOOD_ISBN and validate_each will be created for the classes you import Isbn into (i.e. for Book and BookPart)