Re: select_for_update running on db_for_read

Showing 1-5 of 5 messages
Re: select_for_update running on db_for_read Ioan Alexandru Cucu 2/18/13 7:42 AM
The correct patch would be:
(Set _for_write on the cloned object rather than the original...)

--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -664,6 +664,7 @@ class QuerySet(object):
         # Default to false for nowait
         nowait = kwargs.pop('nowait', False)
         obj = self._clone()
+        obj._for_write = True
         obj.query.select_for_update = True
         obj.query.select_for_update_nowait = nowait
         return obj

Re: select_for_update running on db_for_read Shai Berger 2/19/13 11:23 AM
Hi,

On Monday 18 February 2013, Ioan Alexandru Cucu wrote:
>
> If I'm running a select_for_update statement in a multidb environment that
> uses a read-only slave database
>
Why does this operation make sense?

Why would you select-for-update if you don't intend to update? Or does
updating on a read-only slave somehow make sense?

Thanks,
        Shai.
Re: select_for_update running on db_for_read Ioan Alexandru Cucu 2/19/13 11:44 PM
Hi,

Well that's what I'm saying. It doesn't make any sense.
By default, django runs the select_for_update query on the 'for read' database instead of running it on the 'for write' database.

In order to make my code not to break, I need to explicitly tell django that I want the query to run on the 'for write' database.

=> I need to write:
pages = Page.objects.select_for_update().using(router.db_for_write(Page)).all()
instead of just
pages = Page.objects.select_for_update().all()

If I don't add the 'using(router.db_for_write(Page)' I get the traceback mentioned in my initial message.

Regards,
Alex
Re: select_for_update running on db_for_read Russell Keith-Magee 2/19/13 11:58 PM
Hi Ioan,

I'd need to dig into the code to be 100% certain, but what you've described seems plausible, and the fix looks like it's going in the right direction. A ticket is certainly called for.

As for the patch -- it needs tests :-)

Django's test suite has support for testing multiple-database situations; check the regressiontests/multiple_database test suite for examples of how to use it. SELECT_FOR_UPDATE has it's own test suite - modeltests/select_for_update - so the tests probably belong in there.

Yours,
Russ Magee %-)

On Mon, Feb 18, 2013 at 10:59 PM, Ioan Alexandru Cucu <alexandru...@gmail.com> wrote:
Hi,

I wanted to raise a but around this, but I thought it might be a better idea to ask first on the developer's group.

If I'm running a select_for_update statement in a multidb environment that uses a read-only slave database, I get the following traceback:

Traceback:
File "/home/kux/workspace/src/other/django/django/core/handlers/base.py" in get_response
  111.                         response = callback(request, *callback_args, **callback_kwargs)
File "/home/kux/workspace/src/other/django/django/contrib/admin/options.py" in wrapper
  366.                 return self.admin_site.admin_view(view)(*args, **kwargs)
File "/home/kux/workspace/src/other/django/django/utils/decorators.py" in _wrapped_view
  91.                     response = view_func(request, *args, **kwargs)
File "/home/kux/workspace/src/other/django/django/views/decorators/cache.py" in _wrapped_view_func
  89.         response = view_func(request, *args, **kwargs)
File "/home/kux/workspace/src/other/django/django/contrib/admin/sites.py" in inner
  196.             return view(request, *args, **kwargs)
File "/home/kux/workspace/src/other/django/django/db/transaction.py" in inner
  209.                 return func(*args, **kwargs)
File "/home/kux/workspace/src/other/django-cms/cms/admin/pageadmin.py" in wrap
  154.             Page.objects.db_manager(router.db_for_write(Page))\
File "/home/kux/workspace/src/other/django/django/db/models/query.py" in exists
  562.             return self.query.has_results(using=self.db)
File "/home/kux/workspace/src/other/django/django/db/models/sql/query.py" in has_results
  441.         return bool(compiler.execute_sql(SINGLE))
File "/home/kux/workspace/src/other/django/django/db/models/sql/compiler.py" in execute_sql
  818.         cursor.execute(sql, params)
File "/home/kux/workspace/src/other/django/django/db/backends/util.py" in execute
  40.             return self.cursor.execute(sql, params)
File "/home/kux/workspace/src/other/django/django/db/backends/mysql/base.py" in execute
  114.             return self.cursor.execute(query, args)
File "/home/kux/workspace/envs/hb23/local/lib/python2.7/site-packages/MySQLdb/cursors.py" in execute
  174.             self.errorhandler(self, exc, value)
File "/home/kux/workspace/envs/hb23/local/lib/python2.7/site-packages/MySQLdb/connections.py" in defaulterrorhandler
  36.     raise errorclass, errorvalue

Exception Type: DatabaseError at /admin/cms/page/add/
Exception Value: (1290, 'The MySQL server is running with the --read-only option so it cannot execute this statement')


Looking through the source code I found that django.db.models.query.QuerySet.select_for_update doesn't set the _for_write attribute before cloning the queryset.

Is this intended behaviour?
If not, the following patch would fix the issue:

--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -663,6 +663,7 @@ class QuerySet(object):
         """
         # Default to false for nowait
         nowait = kwargs.pop('nowait', False)
+        self._for_write = True
         obj = self._clone()
         obj.query.select_for_update = True
         obj.query.select_for_update_nowait = nowait

Note: I'm running django 1.4.1

Regards,
Alex

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Re: select_for_update running on db_for_read Ioan Alexandru Cucu 2/26/13 2:02 AM
Hi,


And s/but/bug/ in my initial post :)

Regards,
Alex