From f747b61c20cb2ab9bcbca2b272f3fba80cf39ac6 Mon Sep 17 00:00:00 2001
From: Malcolm Tredinnick <malcolm.tredinnick@gmail.com>
Date: Mon, 5 Jan 2009 11:47:48 +0000
Subject: [PATCH] Nested query support.

This extends previous functionality that allowed passing Query objects as the
rvals to filters. You can now pass QuerySets, which requires less poking at
opaque attributes. See the documentation of the "__in" lookup type for the
details.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9701 bcc190cf-cafb-0310-a4f2-bffc1f526a37
---
 django/db/models/query.py               |  9 ++++++
 django/db/models/sql/query.py           | 12 ++++++++
 docs/ref/models/querysets.txt           | 38 ++++++++++++++++++++-----
 tests/regressiontests/queries/models.py |  7 +++++
 4 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 5b9f504d2f695..2f9f9e24ecf92 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -641,6 +641,15 @@ def _merge_sanity_check(self, other):
         """
         pass
 
+    def as_sql(self):
+        """
+        Returns the internal query's SQL and parameters (as a tuple).
+
+        This is a private (internal) method. The name is chosen to provide
+        uniformity with other interfaces (in particular, the Query class).
+        """
+        obj = self.values("pk")
+        return obj.query.as_nested_sql()
 
 class ValuesQuerySet(QuerySet):
     def __init__(self, *args, **kwargs):
diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 61c1e5155766c..432e5e6332592 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -313,6 +313,18 @@ def as_sql(self, with_limits=True, with_col_aliases=False):
         params.extend(self.extra_params)
         return ' '.join(result), tuple(params)
 
+    def as_nested_sql(self):
+        """
+        Perform the same functionality as the as_sql() method, returning an
+        SQL string and parameters. However, the alias prefixes are bumped
+        beforehand (in a copy -- the current query isn't changed).
+
+        Used when nesting this query inside another.
+        """
+        obj = self.clone()
+        obj.bump_prefix()
+        return obj.as_sql()
+
     def combine(self, rhs, connector):
         """
         Merge the 'rhs' query into the current one (with any 'rhs' effects
diff --git a/docs/ref/models/querysets.txt b/docs/ref/models/querysets.txt
index 3525865ac6fec..2334cf075449a 100644
--- a/docs/ref/models/querysets.txt
+++ b/docs/ref/models/querysets.txt
@@ -1035,12 +1035,27 @@ SQL equivalent::
     SELECT ... WHERE id IN (1, 3, 4);
 
 You can also use a queryset to dynamically evaluate the list of values
-instead of providing a list of literal values. The queryset must be
-reduced to a list of individual values using the ``values()`` method,
-and then converted into a query using the ``query`` attribute::
+instead of providing a list of literal values::
 
-    q = Blog.objects.filter(name__contains='Cheddar').values('pk').query
-    e = Entry.objects.filter(blog__in=q)
+    inner_qs = Blog.objects.filter(name__contains='Cheddar')
+    entries = Entry.objects.filter(blog__in=inner_qs)
+
+This queryset will be evaluated as subselect statement::
+
+    SELECT ... WHERE blog.id IN (SELECT id FROM ... WHERE NAME LIKE '%Cheddar%')
+
+The above code fragment could also be written as follows::
+
+    inner_q = Blog.objects.filter(name__contains='Cheddar').values('pk').query
+    entries = Entry.objects.filter(blog__in=inner_q)
+
+.. versionchanged:: 1.1
+    In Django 1.0, only the latter piece of code is valid.
+
+This second form is a bit less readable and unnatural to write, since it
+accesses the internal ``query`` attribute and requires a ``ValuesQuerySet``.
+If your code doesn't require compatibility with Django 1.0, use the first
+form, passing in a queryset directly.
 
 .. warning::
 
@@ -1048,9 +1063,18 @@ and then converted into a query using the ``query`` attribute::
     It's fine to use it like above, but its API may change between Django
     versions.
 
-This queryset will be evaluated as subselect statement::
+.. admonition:: Performance considerations
 
-    SELECT ... WHERE blog.id IN (SELECT id FROM ... WHERE NAME LIKE '%Cheddar%')
+    Be cautious about using nested queries and understand your database
+    server's performance characteristics (if in doubt, benchmark!). Some
+    database backends, most notably MySQL, don't optimize nested queries very
+    well. It is more efficient, in those cases, to extract a list of values
+    and then pass that into the second query. That is, execute two queries
+    instead of one::
+
+        values = Blog.objects.filter(
+                name__contains='Cheddar').values_list('pk', flat=True)
+        entries = Entry.objects.filter(blog__in=values)
 
 gt
 ~~
diff --git a/tests/regressiontests/queries/models.py b/tests/regressiontests/queries/models.py
index 94f2045fe7e52..a3247bb46090a 100644
--- a/tests/regressiontests/queries/models.py
+++ b/tests/regressiontests/queries/models.py
@@ -1008,6 +1008,13 @@ class PointerB(models.Model):
 # optimise the inner query without losing results.
 >>> Annotation.objects.exclude(tag__children__name="t2")
 [<Annotation: a2>]
+
+Nested queries are possible (although should be used with care, since they have
+performance problems on backends like MySQL.
+
+>>> Annotation.objects.filter(notes__in=Note.objects.filter(note="n1"))
+[<Annotation: a1>]
+
 """}
 
 # In Python 2.3 and the Python 2.6 beta releases, exceptions raised in __len__