from django.db import transaction

from registrasion import models as rego


class CreditNoteController(object):

    def __init__(self, credit_note):
        self.credit_note = credit_note

    @classmethod
    def generate_from_invoice(cls, invoice, value):
        ''' Generates a credit note of the specified value and pays it against
        the given invoice. You need to call InvoiceController.update_status()
        to set the status correctly, if appropriate. '''

        credit_note = rego.CreditNote.objects.create(
            invoice=invoice,
            amount=0-value,  # Credit notes start off as a payment against inv.
            reference="ONE MOMENT",
        )
        credit_note.reference = "Generated credit note %d" % credit_note.id
        credit_note.save()

        return cls(credit_note)

    @transaction.atomic
    def apply_to_invoice(self, invoice):
        ''' Applies the total value of this credit note to the specified
        invoice. If this credit note overpays the invoice, a new credit note
        containing the residual value will be created.

        Raises ValidationError if the given invoice is not allowed to be
        paid.
        '''

        from invoice import InvoiceController  # Circular imports bleh.
        inv = InvoiceController(invoice)
        inv.validate_allowed_to_pay()

        # Apply payment to invoice
        rego.CreditNoteApplication.objects.create(
            parent=self.credit_note,
            invoice=invoice,
            amount=self.credit_note.value,
            reference="Applied credit note #%d" % self.credit_note.id,
        )

        inv.update_status()

    # TODO: Add administration fee generator.