how to include Enumerable in Rails in order to have `blank?` functionality

rails-enumerable_spec.rb

require 'active_support/core_ext/module/delegation'
require 'active_support/core_ext/object/blank'
require 'rspec'

class Membership
  class MembershipCollection
    include Enumerable

    delegate :empty?, :each, to: :@members

    def initialize(*members)
      @members = members.flatten
    end

    def free
      select { |m| m.free? }
    end

    def paid
      select { |m| m.paid? }
    end
  end

  attr_accessor :type

  def free?
    type == 'free'
  end

  def paid?
    type == 'paid'
  end
end

puts "\nMembership without state:"
m = Membership.new
puts m.free?
# => false
puts m.paid?
# => false

puts Membership::MembershipCollection.new(m).free.inspect
# => []

puts Membership::MembershipCollection.new(m).paid.inspect
# => []

puts "\nMembership with 'free' state:"
m.type = 'free'

puts m.free?
# => true
puts m.paid?
#=> false

puts Membership::MembershipCollection.new(m).free.inspect
# => [m]

puts Membership::MembershipCollection.new(m).paid.inspect
# => []

puts "\nMembership with 'paid' state:"
m.type = 'paid'

puts m.free?.inspect
# => false
m.paid?
# => true

puts Membership::MembershipCollection.new(m).free.inspect
# => []

puts Membership::MembershipCollection.new(m).paid.inspect
# => [m]

puts "\n\nRSpec test:"

RSpec.describe Membership::MembershipCollection do
  subject { described_class.new(memberships) }
  let(:free_membership) { Membership.new.tap { |m| m.type = 'free'} }
  let(:paid_membership) { Membership.new.tap { |m| m.type = 'paid'} }

  context 'when no memberships' do
    let(:memberships) { [] }

    it { expect(subject).to be_blank }
    it { expect(subject).to be_empty }
    it { expect(subject.free).to be_empty }
    it { expect(subject.paid).to be_empty }
  end

  context 'when free memberships' do
    let(:memberships) { [free_membership] }

    it { expect(subject).not_to be_blank }
    it { expect(subject).not_to be_empty }
    it { expect(subject.free).to eq([free_membership]) }
    it { expect(subject.paid).to be_empty }
  end

  context 'when paid memberships' do
    let(:memberships) { [paid_membership] }

    it { expect(subject).not_to be_blank }
    it { expect(subject).not_to be_empty }
    it { expect(subject.free).to be_empty }
    it { expect(subject.paid).to eq([paid_membership]) }
  end
end

• http://www.eq8.eu/blogs/28-ruby-enumerable-enumerator-lazy-and-domain-specific-collection-objects
• unfinished article on this https://github.com/equivalent/scrapbook2/blob/master/archive/blogs/2016-07-rails-and-enumerable.md