You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: reference_extractor_spec.rb
    path of file: ./repos/gitlabhq/spec/lib/gitlab
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

require 'spec_helper'

RSpec.describe Gitlab::ReferenceExtractor do
  let_it_be(:project) { create(:project) }

  before do
    project.add_developer(project.creator)
  end

  subject {
    
    def prefixes
      described_class.referrables.each_with_object({}) do |referable, result|
        class_name = referable.to_s.camelize
        klass = class_name.constantize if Object.const_defined?(class_name)

        next unless klass.respond_to?(:reference_prefix)

        prefix = klass.reference_prefix
        result[prefix] ||= []
        result[prefix] << referable
      end
    end