Store subevent data for invoice lines

src/pretix/base/exporters/orderlist.py

@@ -559,6 +559,7 @@ class InvoiceDataExporter(MultiSheetListExporter):
                 _('Tax value'),
                 _('Tax rate'),
                 _('Tax name'),
+                _('Event start date'),
 
                 _('Date'),
                 _('Order code'),
@@ -598,6 +599,7 @@ class InvoiceDataExporter(MultiSheetListExporter):
                     l.tax_value,
                     l.tax_rate,
                     l.tax_name,
+                    date_format(l.event_date_from, "SHORT_DATE_FORMAT") if l.event_date_from else "",
                     date_format(i.date, "SHORT_DATE_FORMAT"),
                     i.order.code,
                     i.order.email,

src/pretix/base/models/invoices.py

@@ -212,6 +212,10 @@ class InvoiceLine(models.Model):
     :type tax_rate: decimal.Decimal
     :param tax_name: The name of the applied tax rate
     :type tax_name: str
+    :param subevent: The subevent this line refers to
+    :type subevent: SubEvent
+    :param event_date_from: Event date of the (sub)event at the time the invoice was created
+    :type event_date_from: datetime
     """
     invoice = models.ForeignKey('Invoice', related_name='lines', on_delete=models.CASCADE)
     position = models.PositiveIntegerField(default=0)
@@ -220,6 +224,8 @@ class InvoiceLine(models.Model):
     tax_value = models.DecimalField(max_digits=10, decimal_places=2, default=Decimal('0.00'))
     tax_rate = models.DecimalField(max_digits=7, decimal_places=2, default=Decimal('0.00'))
     tax_name = models.CharField(max_length=190)
+    subevent = models.ForeignKey('SubEvent', null=True, blank=True, on_delete=models.PROTECT)
+    event_date_from = models.DateTimeField(null=True)
 
     @property
     def net_value(self):

src/pretix/base/services/invoices.py

@@ -142,6 +142,7 @@ def build_invoice(invoice: Invoice) -> Invoice:
             InvoiceLine.objects.create(
                 position=i, invoice=invoice, description=desc,
                 gross_value=p.price, tax_value=p.tax_value,
+                subevent=p.subevent, event_date_from=(p.subevent.date_from if p.subevent else invoice.event.date_from),
                 tax_rate=p.tax_rate, tax_name=p.tax_rule.name if p.tax_rule else ''
             )