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queryset-refactor: Added a note about using already present tables in

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extra(tables=...). This is already a problem in trunk and it's pretty much
impossible to work around in a non-complex way, so it's user beware (it's
usually easy enough to avoid the problems).


git-svn-id: http://code.djangoproject.com/svn/django/branches/queryset-refactor@7453 bcc190cf-cafb-0310-a4f2-bffc1f526a37
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Malcolm Tredinnick 2008-04-24 11:41:29 +00:00
parent 675815d6a1
commit 28b1c9c1c9
1 changed files with 22 additions and 1 deletions
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23
docs/db-api.txt
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@ -649,7 +649,7 @@ but it doesn't really matter. This is your chance to really flaunt your
individualism. individualism.
``values_list(*fields)`` ``values_list(*fields)``
~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~
**New in Django development version** **New in Django development version**
@ -931,6 +931,27 @@ of the arguments is required, but you should use at least one of them.
SELECT * FROM blog_entry WHERE id IN (3, 4, 5, 20); SELECT * FROM blog_entry WHERE id IN (3, 4, 5, 20);
Be careful when using the ``tables`` parameter if you're specifying
tables that are already used in the query. When you add extra tables
via the ``tables`` parameter, Django assumes you want that table included
an extra time, if it is already included. That creates a problem,
since the table name will then be given an alias. If a table appears
multiple times in an SQL statement, the second and subsequent occurrences
must use aliases so the database can tell them apart. If you're
referring to the extra table you added in the extra ``where`` parameter
this is going to cause errors.
Normally you'll only be adding extra tables that don't already appear in
the query. However, if the case outlined above does occur, there are a few
solutions. First, see if you can get by without including the extra table
and use the one already in the query. If that isn't possible, put your
``extra()`` call at the front of the queryset construction so that your
table is the first use of that table. Finally, if all else fails, look at
the query produced and rewrite your ``where`` addition to use the alias
given to your extra table. The alias will be the same each time you
construct the queryset in the same way, so you can rely upon the alias
name to not change.
``order_by`` ``order_by``
If you need to order the resulting queryset using some of the new fields If you need to order the resulting queryset using some of the new fields
or tables you have included via ``extra()`` use the ``order_by`` parameter or tables you have included via ``extra()`` use the ``order_by`` parameter