accept_invitation.rb

class AcceptInvitation
  include Waterfall
  include ActiveModel::Validations

  CENTS_PAID_FOR_AFFILIATION = 100

  validate :ensure_invitation_not_accepted
  
  def initialize(invitation)
    @invitation = invitation
  end
  
  def call
    when_falsy { valid? }
      .dam { errors }
    chain(user: :user) { CreateUserFromInvitation.new(invitation).call }
    chain  do
      CreditUser.new(
        user: invitation.inviter, 
        cents: CENTS_PAID_FOR_AFFILIATION
      ).call
    end
    chain  { invitation.accept }
    when_falsy { invitation.save }
      .dam { invitation.errors }
  end
  
  private
  def ensure_invitation_not_accepted
    if invitation.accepted?
      errors.add :invitation_status, 'Invitation already accepted'
    end
  end
  attr_reader :invitation
end