attendee.rb

class Attendee < ActiveRecord::Base
  devise :invitable, :rememberable, :trackable, :omniauthable,
    :registerable, :database_authenticatable, :recoverable
  has_many :reservations, through: :invites
  has_many :invites, foreign_key: :guest_id
end

attendee_stats.rb

  module PeopleRelatedAssociations
    def add_guest_associations
      self.class_eval do
        has_many :checked_in_attendees, through: :reservations, source: :attendee
      end
    end
  end
class AttendeeStats
  attr_reader :attendee
  def initialize(attendee)
    @attendee = attendee
    @attendee.extend(PeopleRelatedAssociations).add_guest_associations
  end
end

attendee_stats_spec.rb

describe AttendeeStats do
  let!(:attendee_stats) { AttendeeStats.new(attendee) }
  let!(:attendee) { create(:attendee) }

  it "extends attendee with guest associations" do
    attendee_stats.attendee.checked_in_attendees.should == []
  end
end

error.txt

  1) AttendeeStats#people_brought_count extends attendee with guest associations
     Failure/Error: attendee_stats.attendee.checked_in_attendees.should == []
     NoMethodError:
       undefined method `association_class' for nil:NilClass
     # /Users/lunks/.rvm/gems/ruby-1.9.3-p448@wego/gems/activerecord-3.2.13/lib/active_record/associations.rb:160:in `association'
     # /Users/lunks/.rvm/gems/ruby-1.9.3-p448@wego/gems/activerecord-3.2.13/lib/active_record/associations/builder/association.rb:44:in `block in define_readers'
     # ./spec/services/attendee_stats_spec.rb:6:in `block (3 levels) in <top (required)>'