custom unique method for coach arrays

coach.rb

  def self.unique(coaches)
    return coaches if coaches.empty?
    raise ArgumentError, "requires array of coaches as an argument" if(coaches.class != Array || coaches.first.class != Coach)
    coach_ids = []
    unique_coaches = []
    coaches.each do |coach|
      unless coach_ids.include?(coach.id)
        coach_ids << coach.id
        unique_coaches << coach
      end
    end
    return unique_coaches
  end