Detect cases of n+1 caused by Django deferred fields

Author: shaib

nplusone/__init__.py

@@ -1 +1 @@
-__version__ = '1.0.0'
+__version__ = '1.1.0a1'

nplusone/ext/django/patch.py

@@ -17,6 +17,7 @@
     create_reverse_many_to_one_manager,
     create_forward_many_to_many_manager,
 )
+from django.db.models.query_utils import DeferredAttribute
 
 
 def get_worker():
@@ -345,3 +346,37 @@ def getitem_queryset(self, index):
         )
     return original_getitem_queryset(self, index)
 query.QuerySet.__getitem__ = getitem_queryset
+
+
+def parse_refresh_from_db(instance, args, kwargs, context):
+    # Instance passed via partial
+    fields = kwargs.get('fields') or args[0]
+    model = type(instance)
+    return model, to_key(instance), fields[0]
+
+
+original_deferred_attribute_get = DeferredAttribute.__get__
+def deferred_attribute_get(self, instance, cls=None):
+    """
+    DeferredAttribute.__get__() is called when a deferred
+    field is accessed. It may or may not trigger a db query;
+    if it does, it's going to be a refresh_from_db() call
+    So we'll emit a `touch` from there
+    """
+    if instance is None:
+        return self
+    # Refresh-from-db, intenally, calls QuerySet.get() on our
+    # instance. Normally, this would make our instance immune
+    # to further notifications. We don't want that to happen,
+    # so we disable the ignore_load signal within refresh_from_db
+    orig_refresh_from_db = instance.refresh_from_db
+    def refresh_from_db(*args, **kwargs):
+        with signals.ignore(signals.ignore_load):
+            return orig_refresh_from_db(*args, **kwargs)
+    instance.refresh_from_db = signals.signalify(
+        signals.lazy_load,
+        refresh_from_db,
+        parser=functools.partial(parse_refresh_from_db, instance),
+    )
+    return original_deferred_attribute_get(self, instance, cls)
+DeferredAttribute.__get__ = deferred_attribute_get

tests/testapp/testapp/models.py

@@ -26,3 +26,7 @@ class Address(models.Model):
 
 class Hobby(models.Model):
     pass
+
+
+class Medicine(models.Model):
+    name = models.CharField(max_length=20)

tests/testapp/testapp/tests.py

@@ -8,6 +8,7 @@
 from django.conf import settings
 from django.http.request import HttpRequest
 from django.http.response import HttpResponse
+from django.test import override_settings
 
 from nplusone.ext.django.patch import setup_state
 from nplusone.ext.django.middleware import NPlusOneMiddleware
@@ -32,6 +33,7 @@ def objects():
     address = models.Address.objects.create(user=user)
     hobby = models.Hobby.objects.create()
     user.hobbies.add(hobby)
+    medicine = models.Medicine.objects.create(name="Allergix")
     return locals()
 
 
@@ -133,6 +135,23 @@ def test_many_to_many_reverse_prefetch(self, objects, calls):
         assert len(calls) == 0
 
 
+@pytest.mark.django_db
+class TestDeferred:
+
+    def test_deferred(self, objects, calls):
+        medicine = list(models.Medicine.objects.defer('name'))[0]
+        medicine.name
+        assert len(calls) == 1
+        call = calls[0]
+        assert call.objects == (models.Medicine, 'Medicine:1', 'name')
+        assert 'medicine.name' in ''.join(call.frame[4])
+
+    def test_non_deferred(self, objects, calls):
+        medicine = list(models.Medicine.objects.all())[0]
+        medicine.name
+        assert len(calls) == 0
+
+
 @pytest.fixture
 def logger(monkeypatch):
     mock_logger = mock.Mock()
@@ -272,16 +291,22 @@ def test_select_nested_unused(self, objects, client, logger):
         assert any('Pet.user' in call[1] for call in calls)
         assert any('User.occupation' in call[1] for call in calls)
 
+    @override_settings(NPLUSONE_WHITELIST=[{'model': 'testapp.User'}])
     def test_many_to_many_whitelist(self, objects, client, logger):
-        settings.NPLUSONE_WHITELIST = [{'model': 'testapp.User'}]
         client.get('/many_to_many/')
         assert not logger.log.called
 
+    @override_settings(NPLUSONE_WHITELIST=[{'model': 'testapp.*'}])
     def test_many_to_many_whitelist_wildcard(self, objects, client, logger):
-        settings.NPLUSONE_WHITELIST = [{'model': 'testapp.*'}]
         client.get('/many_to_many/')
         assert not logger.log.called
 
+    def test_deferred(self, objects, client, logger):
+        client.get('/deferred/')
+        assert len(logger.log.call_args_list) == 1
+        args = logger.log.call_args[0]
+        assert 'Medicine.name' in args[1]
+
 
 @pytest.mark.django_db
 def test_values(objects, lazy_listener):

tests/testapp/testapp/urls.py

@@ -25,4 +25,5 @@
     url(r'^prefetch_nested_unused/$', views.prefetch_nested_unused),
     url(r'^select_nested/$', views.select_nested),
     url(r'^select_nested_unused/$', views.select_nested_unused),
+    url(r'^deferred/$', views.deferred),
 ]

tests/testapp/testapp/views.py

@@ -127,3 +127,8 @@ def select_nested(request):
 def select_nested_unused(request):
     pets = list(models.Pet.objects.all().select_related('user__occupation'))
     return HttpResponse(pets[0])
+
+
+def deferred(request):
+    meds = list(models.Medicine.objects.defer('name'))
+    return HttpResponse("; ".join(med.name for med in meds))