set.rb

require 'pry'

class MySet
  attr_accessor :objects

  def initialize
    @objects = []
  end

  def count
    i = 0
    @objects.each do |object|
      i += 1
    end
    i
  end

  def add(*objects)
    objects.each do |object|
      @objects << object
    end
    @objects.uniq!
  end

  def union(set)
    union_set = MySet.new
    objects = (@objects + set.objects).uniq!
    union_set.objects = objects
  end
end

set_spec.rb

require_relative '../lib/set'

describe MySet do
  before :each do
    @set = MySet.new
  end

  it "is empty by default" do
    expect(@set.count).to eq 0
  end

  it "can add an object to the set" do
    @set.add("Luna")
    expect(@set.count).to eq 1
  end

  it "can add multiple objects to the set" do
    @set.add("Otto", "Luna")
    expect(@set.count).to eq 2
  end

  it "does not contain duplicates" do
    @set.add("Otto")
    @set.add("Otto")
    expect(@set.count).to eq 1
  end

  it "can combine two sets" do
    set1 = MySet.new
    set1.add("otto", "luna")
    set2 = MySet.new
    set2.add("otto", "mars")
    unionset = set1.union(set2)
    expect(unionset.count).to eq 3
  end
end