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/tests/api/test_orders.py

@@ -2798,3 +2798,222 @@ def test_order_resend_link(token_client, organizer, event, order):
         ), format='json', data={}
     )
     assert resp.status_code == 400
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation(token_client, organizer, event, order, item):
+    op = order.positions.first()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('23.00'),
+        'gross_formatted': '23.00',
+        'name': '',
+        'net': Decimal('23.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_item_with_tax(token_client, organizer, event, order, item, taxrule):
+    item2 = event.items.create(name="Budget Ticket", default_price=23, tax_rule=taxrule)
+    op = order.positions.first()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('23.00'),
+        'gross_formatted': '23.00',
+        'name': '',
+        'net': Decimal('19.33'),
+        'rate': Decimal('19.00'),
+        'tax': Decimal('3.67')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_item_with_variation(token_client, organizer, event, order):
+    item2 = event.items.create(name="Budget Ticket", default_price=23)
+    var = item2.variations.create(default_price=12, value="XS")
+    op = order.positions.first()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+            'variation': var.pk
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('12.00'),
+        'gross_formatted': '12.00',
+        'name': '',
+        'net': Decimal('12.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_subevent(token_client, organizer, event, order, subevent):
+    item2 = event.items.create(name="Budget Ticket", default_price=23)
+    op = order.positions.first()
+    op.subevent = subevent
+    op.save()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+            'subevent': subevent.pk
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('23.00'),
+        'gross_formatted': '23.00',
+        'name': '',
+        'net': Decimal('23.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_subevent_with_override(token_client, organizer, event, order, subevent):
+    item2 = event.items.create(name="Budget Ticket", default_price=23)
+    se2 = event.subevents.create(name="Foobar", date_from=datetime.datetime(2017, 12, 27, 10, 0, 0, tzinfo=UTC))
+    se2.subeventitem_set.create(item=item2, price=12)
+    op = order.positions.first()
+    op.subevent = subevent
+    op.save()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+            'subevent': se2.pk
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('12.00'),
+        'gross_formatted': '12.00',
+        'name': '',
+        'net': Decimal('12.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_voucher_matching(token_client, organizer, event, order, subevent, item):
+    item2 = event.items.create(name="Budget Ticket", default_price=23)
+    q = event.quotas.create(name="Quota")
+    q.items.add(item)
+    q.items.add(item2)
+    voucher = event.vouchers.create(price_mode="set", value=15, quota=q)
+    op = order.positions.first()
+    op.voucher = voucher
+    op.save()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('15.00'),
+        'gross_formatted': '15.00',
+        'name': '',
+        'net': Decimal('15.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_voucher_not_matching(token_client, organizer, event, order, subevent, item):
+    item2 = event.items.create(name="Budget Ticket", default_price=23)
+    q = event.quotas.create(name="Quota")
+    q.items.add(item)
+    voucher = event.vouchers.create(price_mode="set", value=15, quota=q)
+    op = order.positions.first()
+    op.voucher = voucher
+    op.save()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('23.00'),
+        'gross_formatted': '23.00',
+        'name': '',
+        'net': Decimal('23.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_net_price(token_client, organizer, event, order, subevent, item, taxrule):
+    taxrule.price_includes_tax = False
+    taxrule.save()
+    item2 = event.items.create(name="Budget Ticket", default_price=10, tax_rule=taxrule)
+    op = order.positions.first()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('11.90'),
+        'gross_formatted': '11.90',
+        'name': '',
+        'net': Decimal('10.00'),
+        'rate': Decimal('19.00'),
+        'tax': Decimal('1.90')
+    }
+
+
+@pytest.mark.django_db
+def test_orderposition_price_calculation_reverse_charge(token_client, organizer, event, order, subevent, item, taxrule):
+    taxrule.price_includes_tax = False
+    taxrule.eu_reverse_charge = True
+    taxrule.home_country = Country('DE')
+    taxrule.save()
+    order.invoice_address.is_business = True
+    order.invoice_address.vat_id = 'ATU1234567'
+    order.invoice_address.vat_id_validated = True
+    order.invoice_address.country = Country('AT')
+    order.invoice_address.save()
+    item2 = event.items.create(name="Budget Ticket", default_price=10, tax_rule=taxrule)
+    op = order.positions.first()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orderpositions/{}/price_calc/'.format(organizer.slug, event.slug, op.pk),
+        data={
+            'item': item2.pk,
+        }
+    )
+    assert resp.status_code == 200
+    assert resp.data == {
+        'gross': Decimal('10.00'),
+        'gross_formatted': '10.00',
+        'name': '',
+        'net': Decimal('10.00'),
+        'rate': Decimal('0.00'),
+        'tax': Decimal('0.00')
+    }