Bug in count() when string formats in select

[m.gajda@…]

Hi!

I have found a bug in Django ORM. This very rare situation, it occured in my application code. I have created complex query using extra() function on query set. This query contains some formatting strings in "select" clause, which will be filled up using "params" clause.

In this case, query set's count() function fails. The reason is in file django/db/models/query.py, in count() method. This method invokes query, but it completely omits the "SELECT" part of generated query. However - all formatting parameters are still passed to the query executing function. Below the example:

> from django.contrib.auth.models import User

> User.objects.filter( pk = 1 ).extra( select = { "foo" : "%s" } , params = ( "bar" , ) ).count()
...
<type 'exceptions.TypeError'>: not all arguments converted during string formatting

I've have created simple patch fixing this bug. It just counts the number of string formats in "select" clause and removed respecting parameters from "params" clause.

I hope, that you will fix the bug soon.

Best regards,
--
Marcin Gajda

[Malcolm Tredinnick]

Not worth fixing in trunk, since parameters in extra(select=...) don't work reliably there in any case (dictionaries don't maintain their order). Will fix on queryset-refactor, though.

[anonymous]

Hm.. I do not understand. What do you mean, when you write about ordering maintenance of dictionaries? This bug does not depend on any of "select" dictionary property, because it relies on while "select" dictionary, intependently of its elements. It sufficient to have just one "%s" formatting in one of "select" dictionary values to make django ORM fail.

Two minutes ago I have tested this issue of stable Django version (0.96.1) - it fails too. I think - it is a big issue, which does not allow to write complex queries.

[Malcolm Tredinnick]

Putting parameters in extra(select=...) in trunk is entirely unsupported. The parameters are a list, the select dictionary is a dictionary and hence arbitrarily ordered. Thus when you have more than one item expecting parameters, the order of the placeholders in the dictionary is not going to match up with the parameters list. Thus fixing a "bug" for something this isn't supported and is known not to work in almost all cases (it only works in the very special case of one parameter in extra(select=...)) isn't worth it.

Queryset-refactor will be merged into trunk very soon and it will be fixed prior to that merge.

I've just noticed that this is, in fact, already covered in a ticket (#3141), so I'm closing as a dupe of that (it also relies on the fix for #2902, updated in [7340]).