#31639: F objects do not error when referencing transforms.
-------------------------------------+-------------------------------------
Reporter: Baptiste Mispelon | Owner: Srinivas
| Reddy Thatiparthy
Type: Bug | Status: assigned
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
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 Tim Park):
Hey guys - Here is an outline of a brute force solution for raising a
{{{ValueError}}} if a lookup is passed into {{{F()}}}.
**Thought Process**
1. We init the F() object with a parameter {{{name}}}
2. If a lookup value is found in {{{name}}}, raise ValueError
3. Otherwise, continue instantiating the F() object as normal
So the next step is figuring out how to check if a lookup value is
contained in {{{name}}}.
1. Lookup expressions follow the format of {{{<fieldname>__<lookup>}}}
2. First, I wanted to raise the error if {{{name}}} contained two
underscores {{{__}}} but I realized we use double underscores for
referencing foreign keys as shown in our example of
{{{client__bills__issued_on}}}.
2. Per docs linked below, lookups are ALWAYS evaluated last. So if we
split the {{{name}}} string on character {{{__}}}, the last portion of
that split would be a potential lookup expression. Check if that potential
lookup expression exists in Django's actual lookup expressions. I could
follow this process to check for transforms within {{{name}}}
**Conclusion**
So this technically works but it seems hacky because it won't take into
account situations where users define their own custom lookups.
Let me know if I am missing something obvious here. Happy to take
hints/direction to help explore the right places.
Thanks!
**References**
Lookup Positioning Docs [https://docs.djangoproject.com/en/3.0/howto
/custom-lookups/#how-django-determines-the-lookups-and-transforms-which-
are-used]
List of All Django Field Lookups
[https://docs.djangoproject.com/en/3.0/ref/models/querysets/#field-
lookups]
Code for F()
[https://github.com/django/django/blob/58a336a674a658c1cda6707fe1cacb56aaed3008/django/db/models/expressions.py#L560]
{{{
class F(Combinable):
def __init__(self, name):
lookups = {'exact', 'iexact', 'contains', 'icontains', 'in', 'gt',
'gte', 'lt', 'lte', 'startswith', 'istartswith',
'endswith',
'iendswith', 'range', 'date', 'year', 'iso_year',
'month', 'day',
'week', 'week_day', 'quarter', 'time', 'hour',
'minute', 'second',
'isnull', 'regex', 'iregex'}
if name.split("__")[-1] in lookups:
raise ValueError("F() objects do not support lookups")
self.name = name
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/31639#comment:6>
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