From 79653a414857e4f93918051843afbc1f7c9a7f99 Mon Sep 17 00:00:00 2001
From: Malcolm Tredinnick <malcolm.tredinnick@gmail.com>
Date: Fri, 30 Nov 2007 04:44:32 +0000
Subject: [PATCH] queryset-refactor: Implemented filtering by output columns
 specified in extra(select=...). Refs #4002.

git-svn-id: http://code.djangoproject.com/svn/django/branches/queryset-refactor@6762 bcc190cf-cafb-0310-a4f2-bffc1f526a37
---
 django/db/models/sql/query.py           | 12 ++++++++++--
 django/db/models/sql/where.py           |  6 +++++-
 tests/regressiontests/queries/models.py | 13 +++++++++++++
 3 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index 703d7a18b7..907903dc90 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -692,8 +692,16 @@ class Query(object):
         opts = self.model._meta
         alias = self.join((None, opts.db_table, None, None))
 
-        field, target, unused, join_list, nullable = self.setup_joins(parts,
-                opts, alias, (connection == AND))
+        try:
+            field, target, unused, join_list, nullable = self.setup_joins(parts,
+                    opts, alias, (connection == AND))
+        except TypeError, e:
+            if len(parts) != 1 or parts[0] not in self.extra_select:
+                raise e
+            # Filtering on some alias from extra(select=...)
+            self.where.add([None, parts[0], None, lookup_type, value],
+                    connection)
+            return
         col = target.column
         alias = join_list[-1][-1]
 
diff --git a/django/db/models/sql/where.py b/django/db/models/sql/where.py
index 5fa1ae8096..6dc3e2d5ee 100644
--- a/django/db/models/sql/where.py
+++ b/django/db/models/sql/where.py
@@ -5,6 +5,7 @@ import datetime
 
 from django.utils import tree
 from django.db import connection
+from django.db.models.fields import Field
 from datastructures import EmptyResultSet
 
 # Connection types
@@ -96,7 +97,10 @@ class WhereNode(tree.Node):
         else:
             format = '%s %s'
 
-        params = field.get_db_prep_lookup(lookup_type, value)
+        if field:
+            params = field.get_db_prep_lookup(lookup_type, value)
+        else:
+            params = Field().get_db_prep_lookup(lookup_type, value)
 
         if lookup_type in connection.operators:
             return (format % (field_sql,
diff --git a/tests/regressiontests/queries/models.py b/tests/regressiontests/queries/models.py
index fe0940152d..ea87a2fd41 100644
--- a/tests/regressiontests/queries/models.py
+++ b/tests/regressiontests/queries/models.py
@@ -404,5 +404,18 @@ thus fail.)
 ...     params.reverse()
 >>> Item.objects.extra(select=SortedDict(s), params=params).values('a','b')[0]
 {'a': u'one', 'b': u'two'}
+
+Bug #4002
+Attributes used in extra(select=...) are available for use in subsequent
+order_by() and filter() calls.
+
+# Order by the number of tags attached to an item.
+>>> l = Item.objects.extra(select={'count': 'select count(*) from queries_item_tags where queries_item_tags.item_id = queries_item.id'}).order_by('-count')
+>>> [o.count for o in l]
+[2, 2, 1, 0]
+
+# Filter those items that have exactly one tag attacjed.
+>>> Item.objects.extra(select={'count': 'select count(*) from queries_item_tags where queries_item_tags.item_id = queries_item.id'}).filter(count=1)
+[<Item: four>]
 """}