Change semantics of changing orders (#1260)

* Change semantics of changing orders

This basically does two things to the "Change products" view of orders and the
OrderChangeManager program API:

1) It decouples changing items or subevents from changing prices.
   OrderChangeManager.change_item() and .change_subevent() no longer
   touch the price of a position. Instead .change_price() needs to be
   called explicitly. However, a client-side JavaScript component now
   *proposes* a new price based on the changed item or subevent.

2) The user interface now exposes the possibility of doing multiple
   things at the same time, i.e. changing the item, subevent and price
   in the same operation. OrderChangeManager already allowed this
   before.

(1) is basically a consequence of (2), while (2) is a prerequesite for
e.g. the `seating` branch, where changing the subevent will always
require changing the seat.

* Add tests for price calculation API

src/pretix/control/forms/orders.py

@@ -9,9 +9,7 @@ from django.utils.timezone import now
 from django.utils.translation import pgettext_lazy, ugettext_lazy as _
 
 from pretix.base.forms import I18nModelForm, PlaceholderValidator
-from pretix.base.models import (
-    InvoiceAddress, Item, ItemAddOn, Order, OrderPosition,
-)
+from pretix.base.models import InvoiceAddress, ItemAddOn, Order, OrderPosition
 from pretix.base.models.event import SubEvent
 from pretix.base.services.pricing import get_price
 from pretix.control.forms.widgets import Select2
@@ -150,15 +148,6 @@ class CommentForm(I18nModelForm):
         }
 
 
-class SubEventChoiceField(forms.ModelChoiceField):
-    def label_from_instance(self, obj):
-        p = get_price(self.instance.item, self.instance.variation,
-                      voucher=self.instance.voucher,
-                      subevent=obj)
-        return '{} – {} ({})'.format(obj.name, obj.get_date_range_display(),
-                                     p.print(self.instance.order.event.currency))
-
-
 class OtherOperationsForm(forms.Form):
     recalculate_taxes = forms.BooleanField(
         label=_('Re-calculate taxes'),
@@ -265,12 +254,13 @@ class OrderPositionAddForm(forms.Form):
 
 
 class OrderPositionChangeForm(forms.Form):
-    itemvar = forms.ChoiceField()
-    subevent = SubEventChoiceField(
+    itemvar = forms.ChoiceField(
+        required=False,
+    )
+    subevent = forms.ModelChoiceField(
         SubEvent.objects.none(),
-        label=pgettext_lazy('subevent', 'New date'),
-        required=True,
-        empty_label=None
+        required=False,
+        empty_label=_('(Unchanged)')
     )
     price = forms.DecimalField(
         required=False,
@@ -278,53 +268,49 @@ class OrderPositionChangeForm(forms.Form):
         localize=True,
         label=_('New price (gross)')
     )
-    operation = forms.ChoiceField(
+    operation_secret = forms.BooleanField(
         required=False,
-        widget=forms.RadioSelect,
-        choices=(
-            ('product', 'Change product'),
-            ('price', 'Change price'),
-            ('subevent', 'Change event date'),
-            ('cancel', 'Remove product'),
-            ('split', 'Split into new order'),
-            ('secret', 'Regenerate secret'),
-        )
+        label=_('Generate a new secret')
+    )
+    operation_cancel = forms.BooleanField(
+        required=False,
+        label=_('Cancel this position')
+    )
+    operation_split = forms.BooleanField(
+        required=False,
+        label=_('Split into new order')
     )
-    change_product_keep_price = forms.BooleanField(required=False)
 
     def __init__(self, *args, **kwargs):
         instance = kwargs.pop('instance')
         initial = kwargs.get('initial', {})
 
-        try:
-            ia = instance.order.invoice_address
-        except InvoiceAddress.DoesNotExist:
-            ia = None
-
-        if instance:
-            try:
-                if instance.variation:
-                    initial['itemvar'] = '%d-%d' % (instance.item.pk, instance.variation.pk)
-                elif instance.item:
-                    initial['itemvar'] = str(instance.item.pk)
-            except Item.DoesNotExist:
-                pass
-
-            if instance.item.tax_rule and not instance.item.tax_rule.price_includes_tax:
-                initial['price'] = instance.price - instance.tax_value
-            else:
-                initial['price'] = instance.price
-        initial['subevent'] = instance.subevent
+        if instance.item.tax_rule and not instance.item.tax_rule.price_includes_tax:
+            initial['price'] = instance.price - instance.tax_value
+        else:
+            initial['price'] = instance.price
 
         kwargs['initial'] = initial
         super().__init__(*args, **kwargs)
         if instance.order.event.has_subevents:
-            self.fields['subevent'].instance = instance
             self.fields['subevent'].queryset = instance.order.event.subevents.all()
+            self.fields['subevent'].widget = Select2(
+                attrs={
+                    'data-model-select2': 'event',
+                    'data-select2-url': reverse('control:event.subevents.select2', kwargs={
+                        'event': instance.order.event.slug,
+                        'organizer': instance.order.event.organizer.slug,
+                    }),
+                    'data-placeholder': _('(Unchanged)')
+                }
+            )
+            self.fields['subevent'].widget.choices = self.fields['subevent'].choices
         else:
             del self.fields['subevent']
 
-        choices = []
+        choices = [
+            ('', _('(Unchanged)'))
+        ]
         for i in instance.order.event.items.prefetch_related('variations').all():
             pname = str(i)
             if not i.is_available():
@@ -333,14 +319,10 @@ class OrderPositionChangeForm(forms.Form):
 
             if variations:
                 for v in variations:
-                    p = get_price(i, v, voucher=instance.voucher, subevent=instance.subevent,
-                                  invoice_address=ia)
                     choices.append(('%d-%d' % (i.pk, v.pk),
-                                    '%s – %s (%s)' % (pname, v.value, p.print(instance.order.event.currency))))
+                                    '%s – %s' % (pname, v.value)))
             else:
-                p = get_price(i, None, voucher=instance.voucher, subevent=instance.subevent,
-                              invoice_address=ia)
-                choices.append((str(i.pk), '%s (%s)' % (pname, p.print(instance.order.event.currency))))
+                choices.append((str(i.pk), pname))
         self.fields['itemvar'].choices = choices
         change_decimal_field(self.fields['price'], instance.order.event.currency)