Fixes #2737 -- Added code to allow None as a query value for __exact queries, raising an error otherwise. __exact=None is interpreted as the SQL 'value = NULL'. This fixes some minor problems with queries on unsaved objects with related object sets, and stops queries with a value of None being outright ignored (even if they reference an unknown attribute).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3902 bcc190cf-cafb-0310-a4f2-bffc1f526a37

1 files changed, 16 insertions, 5 deletions

diff --git a/docs/db-api.txt b/docs/db-api.txt
index 0d1f049601..2f0c8b0589 100644
--- a/docs/db-api.txt
+++ b/docs/db-api.txt
@@ -876,15 +876,18 @@ The database API supports the following lookup types:
 exact
 ~~~~~
 
-Exact match.
+Exact match. If the value provided for comparison is ``None``, it will 
+be interpreted as an SQL ``NULL`` (See isnull_ for more details).  
 
-Example::
+Examples::
 
     Entry.objects.get(id__exact=14)
+    Entry.objects.get(id__exact=None)
 
-SQL equivalent::
+SQL equivalents::
 
     SELECT ... WHERE id = 14;
+    SELECT ... WHERE id = NULL;
 
 iexact
 ~~~~~~
@@ -1103,8 +1106,8 @@ such as January 3, July 3, etc.
 isnull
 ~~~~~~
 
-``NULL`` or ``IS NOT NULL`` match. Takes either ``True`` or ``False``, which
-correspond to ``IS NULL`` and ``IS NOT NULL``, respectively.
+Takes either ``True`` or ``False``, which correspond to SQL queries of 
+``IS NULL`` and ``IS NOT NULL``, respectively.
 
 Example::
 
@@ -1114,6 +1117,14 @@ SQL equivalent::
 
     SELECT ... WHERE pub_date IS NULL;
 
+.. admonition:: ``__isnull=True`` vs ``__exact=None``
+
+    There is an important difference between ``__isnull=True`` and 
+    ``__exact=None``. ``__exact=None`` will *always* return an empty result
+    set, because SQL requires that no value is equal to ``NULL``. 
+    ``__isnull`` determines if the field is currently holding the value 
+    of ``NULL`` without performing a comparison.
+
 search
 ~~~~~~