actually add the model test

inventory/tests/test_models.py

@@ -0,0 +1,23 @@
+# -*- mode: python -*-
+
+import pytest
+
+from django.contrib.auth import get_user_model
+from inventory.models import Item, Order, OrderItem, Vendor, Unit, Category, Account
+
+def get_sentinel_user():
+    return get_user_model().objects.get_or_create(username="deleted")[0]
+
+@pytest.mark.django_db
+def test_order_cost():
+    user = get_sentinel_user()
+    category = Category.objects.create(name="glues and pastes")
+    unit = Unit.objects.create(name="each")
+    vendor = Vendor.objects.create(name="Unicorn Dispensary")
+    account = Account.objects.create(code="1234", description="unicorn paste fund")
+    item = Item.objects.create(name="unicorn paste", category=category, unit=unit, vendor=vendor, catalog="UPASTE1")
+    order = Order.objects.create(name="yearly unicorn paste supply", account=account, ordered_by=user)
+
+    orderitem = OrderItem.objects.create(item=item, order=order, units_purchased=10, cost=20)
+
+    assert orderitem.total_price() == 200