simple_spec.rb

require 'spec_helper'
require 'dry-validation'

module AllowList
  NAMES = %w(john jane)
end

class Admin
  SCHEMA = Dry::Validation.Params do
    configure do
      def allow_list
        AllowList::NAMES
      end
    end

    required(:name).filled?(included_in?: allow_list)
  end

  def initialize(name)
    @name = name
  end

  def valid?
    SCHEMA.call({ name: @name }).success?
  end
end

RSpec.describe Admin do
  describe "without a stubbed list" do
    it "validates using original list" do
      stub_const("AllowList::NAMES", %w(peter))
      jane = Admin.new("jane")
      john = Admin.new("john")
      peter = Admin.new("peter")

      expect(jane).to be_valid
      expect(john).to be_valid
      expect(peter).not_to be_valid
    end
  end

  describe "with stubbed allow list" do
    it "validates using stubbed list" do
      stub_const("AllowList::NAMES", %w(peter))
      jane = Admin.new("jane")
      peter = Admin.new("peter")

      expect(jane).not_to be_valid
      expect(peter).to be_valid
    end
  end
end