#17664: Smart `if` tag silencing exceptions plus `QuerySet` caching equals buggy
behaviour.
-------------------------------------+-------------------------------------
Reporter: mrmachine | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: SVN
Severity: Normal | Resolution:
Keywords: smart if tag | Triage Stage:
queryset exception silenced | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by krzysiumed):
mrmachine, look at example:
{{{
>>> invalid_qs = Book.objects.order_by('invalid_field') # no error occures
>>> type(invalid_qs)
<class 'django.db.models.query.QuerySet'>
>>> bool(invalid_qs) # this will raise exception
Traceback (most recent call last):
(... traceback ...)
FieldError: Cannot resolve keyword 'invalid_field' into field. Choices
are: id, membership, name, part
}}}
The queryset is evaluated in both situations -- `{% if qs %}` as well as
`{% if not qs %}`, but it is evaluated in different places in python code.
Queryset from `{% if qs %}` is evaluated here:
https://code.djangoproject.com/browser/django/trunk/django/template/defaulttags.py#L280
and this line raises our `FieldError`. There is no try-except so the
exception is not caught and we can see traceback.
Queryset from `{% if not qs %}` is evaluated here:
https://code.djangoproject.com/browser/django/trunk/django/template/smartif.py#L98
(and line 83). Line 83:
{{{
return func(context, self.first)
}}}
calls lambda function from line 98:
{{{
lambda context, x: not x.eval(context))
}}}
`x.eval(context)` is our `QuerySet`. Next, we want to negate the queryset
because of `not` operator, so `bool(queryset)` is computed and it raises
exception. We back to line 83 of `smartif.py`, where the lambda-function
was called. As you can see, there is `except` clause and it catches every
exception including our `FieldError`.
--
Ticket URL: <https://code.djangoproject.com/ticket/17664#comment:4>
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