Prevent unnecessary database query in UseFirstStockRecord strategy when passed an annotated product.

src/oscar/apps/partner/strategy.py

 # Mixins - these can be used to construct the appropriate strategy class
 
 
-class UseFirstStockRecord(object):
+class UseFirstStockRecord:
     """
     Stockrecord selection mixin for use with the ``Structured`` base strategy.
     This mixin picks the first (normally only) stockrecord to fulfil a product.
-
-    This is backwards compatible with Oscar<0.6 where only one stockrecord per
-    product was permitted.
     """
 
     def select_stockrecord(self, product):
-        return product.stockrecords.first()
+        # We deliberately fetch by index here, to ensure that no additional database queries are made
+        # when stockrecords have already been prefetched in a queryset annotated using ProductQuerySet.base_queryset
+        try:
+            return product.stockrecords.all()[0]
+        except IndexError:
+            pass
 
 
 class StockRequired(object):

tests/integration/partner/test_selector_mixin.py

 from django.test import TestCase
 
+from oscar.apps.catalogue.models import Product
 from oscar.apps.partner import strategy
 from oscar.test import factories
 
 
     def test_returns_none_when_no_stock_records(self):
         self.assertIsNone(self.mixin.select_stockrecord(self.product))
+
+    def test_does_not_generate_additional_query_when_passed_product_from_base_queryset(self):
+        product = Product.objects.base_queryset().first()
+        # Regression test for https://github.com/django-oscar/django-oscar/issues/3875
+        # If passed a product from a queryset annotated by base_queryset, then
+        # the selector should not trigger any additional database queries because
+        # it should rely on the prefetched stock records.
+        with self.assertNumQueries(0):
+            self.mixin.select_stockrecord(product)