gistfile1.rb

  def add_organizer(user)
    raise ArgumentError unless user.is_a?(User)
    part = participations.new
    part.user = user
    part.organizer = true
    part.save!
  end
  
  def add_speaker(user)
    raise ArgumentError unless user.is_a?(User)
    part = participations.new
    part.user = user
    part.speaker = true
    part.save!
  end
  
  def add_participant(user)
    raise ArgumentError unless user.is_a?(User)
    participants << user unless has_participant?(user)
  end
  
  def has_participant?(user)
    raise ArgumentError unless user.is_a?(User)
    participants.include?(user)
  end