class ActiveModel::OrderedSet

def initialize(array)
  @array = array
  @hash = {}
  array.each do |item|
    @hash[item] = true
  end
end

def merge!(other)
  other.each do |item|
    next if @hash.key?(item)
    @hash[item] = true
    @array.push item
  end
end

def to_a
  @array
end