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

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
 ~~