Fake set implementation in Ruby

fake_set.rb

# filename: spec/fake_set_spec.rb
require 'spec_helper'
require 'fake_set'

describe FakeSet, '#union' do
  it "unions elements of two set" do
    set1 = FakeSet.new([1, 2, 3])
    set2 = FakeSet.new([3, 4, 5])

    expect(set1.union(set2).data).to eq [1, 2, 3, 4, 5]
  end
end

# filename: lib/fake_set.rb
class FakeSet
  def initialize(*elements)
    @data = elements
  end

  def <<(new_element)
    @data << new_element unless @data.index(new_element)

    self
  end

  def each
    if block_given?
      @data.each {|e| yield e }
    else
      Enumerator.new(self, :each)
    end
  end

  def data
    @data.dup
  end

  def union(set)
    set.each do |el|
      self << el
    end

    self
  end
end