From d068ad0c01be9bb085c86a4e0c4dd047a85ce18d Mon Sep 17 00:00:00 2001
From: Malcolm Tredinnick <malcolm.tredinnick@gmail.com>
Date: Thu, 8 Jan 2009 05:16:21 +0000
Subject: [PATCH] Using querysets as an rvalue in filter() calls was causing an
 unnecessary database query, due to a bool() call. This change stops that
 behaviour.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9715 bcc190cf-cafb-0310-a4f2-bffc1f526a37
---
 django/db/models/query.py               | 8 ++++++++
 django/db/models/sql/where.py           | 2 ++
 tests/regressiontests/queries/models.py | 7 +++++++
 3 files changed, 17 insertions(+)

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 2f9f9e24ec..563b902bb5 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -651,6 +651,10 @@ class QuerySet(object):
         obj = self.values("pk")
         return obj.query.as_nested_sql()
 
+    # When used as part of a nested query, a queryset will never be an "always
+    # empty" result.
+    value_annotation = True
+
 class ValuesQuerySet(QuerySet):
     def __init__(self, *args, **kwargs):
         super(ValuesQuerySet, self).__init__(*args, **kwargs)
@@ -795,6 +799,10 @@ class EmptyQuerySet(QuerySet):
         # (it raises StopIteration immediately).
         yield iter([]).next()
 
+    # EmptyQuerySet is always an empty result in where-clauses (and similar
+    # situations).
+    value_annotation = False
+
 
 def get_cached_row(klass, row, index_start, max_depth=0, cur_depth=0,
                    requested=None):
diff --git a/django/db/models/sql/where.py b/django/db/models/sql/where.py
index a9fca7df11..9ce1e7bf2d 100644
--- a/django/db/models/sql/where.py
+++ b/django/db/models/sql/where.py
@@ -66,6 +66,8 @@ class WhereNode(tree.Node):
         # here in the future (using Python types is suggested for consistency).
         if isinstance(value, datetime.datetime):
             annotation = datetime.datetime
+        elif hasattr(value, 'value_annotation'):
+            annotation = value.value_annotation
         else:
             annotation = bool(value)
 
diff --git a/tests/regressiontests/queries/models.py b/tests/regressiontests/queries/models.py
index a3247bb460..05842ebe7b 100644
--- a/tests/regressiontests/queries/models.py
+++ b/tests/regressiontests/queries/models.py
@@ -1015,6 +1015,13 @@ performance problems on backends like MySQL.
 >>> Annotation.objects.filter(notes__in=Note.objects.filter(note="n1"))
 [<Annotation: a1>]
 
+Nested queries should not evaluate the inner query as part of constructing the
+SQL. This test verifies this: if the inner query is evaluated, the outer "in"
+lookup will raise an EmptyResultSet exception (as the inner query returns
+nothing).
+>>> print Annotation.objects.filter(notes__in=Note.objects.filter(note="xyzzy")).query
+SELECT ...
+
 """}
 
 # In Python 2.3 and the Python 2.6 beta releases, exceptions raised in __len__