Fixed #19173 -- Made EmptyQuerySet a marker class only

The guarantee that no queries will be made when accessing results is
done by new EmptyWhere class which is used for query.where and having.

Thanks to Simon Charette for reviewing and valuable suggestions.

4 files changed, 39 insertions, 31 deletions

diff --git a/tests/modeltests/basic/tests.py b/tests/modeltests/basic/tests.py
index 1c83b980a7..dba9a686d9 100644
--- a/tests/modeltests/basic/tests.py
+++ b/tests/modeltests/basic/tests.py
@@ -4,6 +4,7 @@ from datetime import datetime
 
 from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
 from django.db.models.fields import Field, FieldDoesNotExist
+from django.db.models.query import EmptyQuerySet
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
 from django.utils.translation import ugettext_lazy
@@ -639,3 +640,9 @@ class ModelTest(TestCase):
         Article.objects.bulk_create([Article(headline=lazy, pub_date=datetime.now())])
         article = Article.objects.get()
         self.assertEqual(article.headline, notlazy)
+
+    def test_emptyqs(self):
+        # Can't be instantiated
+        with self.assertRaises(TypeError):
+            EmptyQuerySet()
+        self.assertTrue(isinstance(Article.objects.none(), EmptyQuerySet))
diff --git a/tests/modeltests/get_object_or_404/tests.py b/tests/modeltests/get_object_or_404/tests.py
index 3b234c6cd3..38ebeb4f8c 100644
--- a/tests/modeltests/get_object_or_404/tests.py
+++ b/tests/modeltests/get_object_or_404/tests.py
@@ -53,7 +53,7 @@ class GetObjectOr404Tests(TestCase):
             get_object_or_404, Author.objects.all()
         )
 
-        # Using an EmptyQuerySet raises a Http404 error.
+        # Using an empty QuerySet raises a Http404 error.
         self.assertRaises(Http404,
             get_object_or_404, Article.objects.none(), title__contains="Run"
         )
diff --git a/tests/modeltests/lookup/tests.py b/tests/modeltests/lookup/tests.py
index 98358e3d10..de7105f92d 100644
--- a/tests/modeltests/lookup/tests.py
+++ b/tests/modeltests/lookup/tests.py
@@ -436,7 +436,7 @@ class LookupTests(TestCase):
             ])
 
     def test_none(self):
-       # none() returns an EmptyQuerySet that behaves like any other QuerySet object
+       # none() returns a QuerySet that behaves like any other QuerySet object
         self.assertQuerysetEqual(Article.objects.none(), [])
         self.assertQuerysetEqual(
             Article.objects.none().filter(headline__startswith='Article'), [])
diff --git a/tests/regressiontests/queries/tests.py b/tests/regressiontests/queries/tests.py
index e3e515025c..7d01c16255 100644
--- a/tests/regressiontests/queries/tests.py
+++ b/tests/regressiontests/queries/tests.py
@@ -9,7 +9,7 @@ from django.conf import settings
 from django.core.exceptions import FieldError
 from django.db import DatabaseError, connection, connections, DEFAULT_DB_ALIAS
 from django.db.models import Count, F, Q
-from django.db.models.query import ITER_CHUNK_SIZE, EmptyQuerySet
+from django.db.models.query import ITER_CHUNK_SIZE
 from django.db.models.sql.where import WhereNode, EverythingNode, NothingNode
 from django.db.models.sql.datastructures import EmptyResultSet
 from django.test import TestCase, skipUnlessDBFeature
@@ -663,31 +663,32 @@ class Queries1Tests(BaseQuerysetTest):
             Item.objects.filter(created__in=[self.time1, self.time2]),
             ['<Item: one>', '<Item: two>']
         )
-
     def test_ticket7235(self):
         # An EmptyQuerySet should not raise exceptions if it is filtered.
-        q = EmptyQuerySet()
-        self.assertQuerysetEqual(q.all(), [])
-        self.assertQuerysetEqual(q.filter(x=10), [])
-        self.assertQuerysetEqual(q.exclude(y=3), [])
-        self.assertQuerysetEqual(q.complex_filter({'pk': 1}), [])
-        self.assertQuerysetEqual(q.select_related('spam', 'eggs'), [])
-        self.assertQuerysetEqual(q.annotate(Count('eggs')), [])
-        self.assertQuerysetEqual(q.order_by('-pub_date', 'headline'), [])
-        self.assertQuerysetEqual(q.distinct(), [])
-        self.assertQuerysetEqual(
-            q.extra(select={'is_recent': "pub_date > '2006-01-01'"}),
-            []
-        )
-        q.query.low_mark = 1
-        self.assertRaisesMessage(
-            AssertionError,
-            'Cannot change a query once a slice has been taken',
-            q.extra, select={'is_recent': "pub_date > '2006-01-01'"}
-        )
-        self.assertQuerysetEqual(q.reverse(), [])
-        self.assertQuerysetEqual(q.defer('spam', 'eggs'), [])
-        self.assertQuerysetEqual(q.only('spam', 'eggs'), [])
+        Eaten.objects.create(meal='m')
+        q = Eaten.objects.none()
+        with self.assertNumQueries(0):
+            self.assertQuerysetEqual(q.all(), [])
+            self.assertQuerysetEqual(q.filter(meal='m'), [])
+            self.assertQuerysetEqual(q.exclude(meal='m'), [])
+            self.assertQuerysetEqual(q.complex_filter({'pk': 1}), [])
+            self.assertQuerysetEqual(q.select_related('food'), [])
+            self.assertQuerysetEqual(q.annotate(Count('food')), [])
+            self.assertQuerysetEqual(q.order_by('meal', 'food'), [])
+            self.assertQuerysetEqual(q.distinct(), [])
+            self.assertQuerysetEqual(
+                q.extra(select={'foo': "1"}),
+                []
+            )
+            q.query.low_mark = 1
+            self.assertRaisesMessage(
+                AssertionError,
+                'Cannot change a query once a slice has been taken',
+                q.extra, select={'foo': "1"}
+            )
+            self.assertQuerysetEqual(q.reverse(), [])
+            self.assertQuerysetEqual(q.defer('meal'), [])
+            self.assertQuerysetEqual(q.only('meal'), [])
 
     def test_ticket7791(self):
         # There were "issues" when ordering and distinct-ing on fields related
@@ -1935,8 +1936,8 @@ class CloneTests(TestCase):
 
 class EmptyQuerySetTests(TestCase):
     def test_emptyqueryset_values(self):
-        # #14366 -- Calling .values() on an EmptyQuerySet and then cloning that
-        # should not cause an error"
+        # #14366 -- Calling .values() on an empty QuerySet and then cloning
+        # that should not cause an error
         self.assertQuerysetEqual(
             Number.objects.none().values('num').order_by('num'), []
         )
@@ -1952,9 +1953,9 @@ class EmptyQuerySetTests(TestCase):
         )
 
     def test_ticket_19151(self):
-        # #19151 -- Calling .values() or .values_list() on an EmptyQuerySet
-        # should return EmptyQuerySet and not cause an error.
-        q = EmptyQuerySet()
+        # #19151 -- Calling .values() or .values_list() on an empty QuerySet
+        # should return an empty QuerySet and not cause an error.
+        q = Author.objects.none()
         self.assertQuerysetEqual(q.values(), [])
         self.assertQuerysetEqual(q.values_list(), [])