Unclear behavior when competition's currency changes when a payment has been made

[viroulep]

Just thought about this: I don't think we handle well the case where the organizer or delegate change the currency when a payment has been made!
The refund behavior is well defined since we refund a Stripe charge, however we don't take into account multiple currencies in some part of the website, eg in paid_entry_fees:

  def paid_entry_fees
    Money.new(
      registration_payments.sum(:amount_lowest_denomination),
      # FIXME: see https://github.com/thewca/worldcubeassociation.org/issues/1257
      (competition.currency_code.blank? ? Money.default_currency : competition.currency_code),
    )
  end

Note how we just make a sum, assuming everything is in the same currency.

Suggestion: make the currency immutable as long as anyone has a non zero balance for the competition.

[larspetrus]

Yeah, I agree with immutability!