limit_choices_to on a ForeignKey can render duplicate options in formfield

[Chris.Wesseling@…]

If you pass a Q object as limit_choices_to on a ForeignKey field involving a join, you may end up with duplicate options in your form.

See regressiontest in patch for a clear view on the problem.

[anonymous]

Replying to SmileyChris:

I've updated the patch to resolve the conflicts I've had since you flagged this one as "Ready for checkin".
No real change.

[Chris.Wesseling@…]

update resolving conflict

[Chris.Wesseling@…]

Is there something I can do to get this checked in? I re-read the ​Triage docs. As far as I can see "A developer checks in the fix" is the only step left.

[Chris Beaven]

The ​1.2 roadmap shows that we're in a feature freeze. I'd suggest bringing this up on the django-dev google group a week or so after 1.2 final is released.

[Luke Plant]

In [15607]:

Fixed #11707 - limit_choices_to on a ForeignKey can render duplicate options in formfield

Thanks to Chris Wesseling for the report and patch.

[Luke Plant]

In [15610]:

[1.2.X] Fixed #11707 - limit_choices_to on a ForeignKey can render duplicate options in formfield

Thanks to Chris Wesseling for the report and patch.

Backport of [15607] from trunk.

[Luke Plant]

In [15791]:

Fixed #15559 - distinct queries introduced by [15607] cause errors with some custom model fields

This patch just reverts [15607] until a more satisfying solution can be
found.

Refs #11707

[Luke Plant]

In [15792]:

[1.2.X] Fixed #15559 - distinct queries introduced by [15607] cause errors with some custom model fields

This patch just reverts [15607] until a more satisfying solution can be
found.

Refs #11707

Backport of [15791] from trunk.

[Luke Plant]

Re-opened due to the fix being reverted, as above. For future reference, a possible alternative solution might be to do filtering of duplicates in Python, at the point of rendering the form field, rather than in the database.

[Chris Wesseling]

Replying to lukeplant:

(The changeset message doesn't reference this ticket)

Can someone point me to an example of such a custom model field or, even better, a test showing the breakage?

Replying to lukeplant:

For future reference, a possible alternative solution might be to do filtering of duplicates in Python, at the point of rendering the form field, rather than in the database.

Assuming 'limit_choices_to' is only used by Forms...

[anonymous]

Replying to charstring:

Replying to lukeplant:

(The changeset message doesn't reference this ticket)

Can someone point me to an example of such a custom model field or, even better, a test showing the breakage?

The discussion linked from the description of the other ticket has an example. It's in dpaste so may not be long-lived. Copying here for reference:

class PointField(models.Field):
    description = _("A geometric point")

    __metaclass__ = models.SubfieldBase

    pattern = re.compile('^\(([\d\.]+),([\d\.]+)\)$')

    def db_type(self, connection):
        if connection.settings_dict['ENGINE'] is not 'django.db.backends.postgresql_psycopg2':
            return None

        return 'point'

    def to_python(self, value):
        if isinstance(value, tuple):
            return (float(value[0]), float(value[1]))

        if not value:
            return (0, 0)

        match = self.pattern.findall(value)[0]
        return (float(match[0]), float(match[1]))

    def get_prep_value(self, value):
        return self.to_python(value)

    def get_db_prep_value(self, value, connection, prepared=False):
        # Casts dates into the format expected by the backend
        if not prepared:
            value = self.get_prep_value(value)
        return '({0}, {1})'.format(value[0], value[1])

    def get_prep_lookup(self, lookup_type, value):
        raise TypeError('Lookup type %r not supported.' % lookup_type)

    def value_to_string(self, obj):
        value = self._get_val_from_obj(obj)
        return self.get_db_prep_value(value)

[Simon Meers]

This is nasty because it not only renders duplicates but also blows up when .get() is called on the queryset if you select one of the duplicates (MultipleObjectsReturned).

[Chris Wesseling]

Talked to Russ. Picked one of the unclean solutions:

filter in python before displaying and checking again before getting the choice.

Thanks to Jonas and Roald!

[Chris Wesseling]

just removed a the previous fix from the comments

[Simon Litchfield]

This issue also breaks ModelChoiceField - MultipleObjectsReturned error

[Chris Wesseling]

Replying to simon29:

This issue also breaks ModelChoiceField - MultipleObjectsReturned error

By "this issue also breaks", do you mean, you've tried the patch and it needs improvement?

If it does work, please set it to "ready for checkin".

[Chris Wesseling]

backported to 1.2.X and refactored to reduce complexity

[Chris Wesseling]

Refactored less complex against trunk

[Chris Wesseling]

against 1.3.X branch

[Jacob]

Discussion from IRC:

[02:24am] I don't see a test case here that emulates the failures seen when the previous (committed then reverted) approach. Am I just missing it?
[09:26am] jacobkm: I also can't say I'm particularly happy with the patch, particularly iterating over the qs in distinct_choices().
[09:26am] chars:It's pretty hard to test for me. It's a case where Postgres can't compare the values.
[09:26am] chars: So it can't test for uniqueness
[09:26am] jacobkm: It also needs additions to documentation to mention that Q() objects are acceptable in limit_choices_to.

[anonymous]

Replying to jacob:

Discussion from IRC:

[09:26am] jacobkm: It also needs additions to documentation to mention that Q() objects are acceptable in limit_choices_to.

​Documentation on ForeignKey.limit_choices_to already mentions: "Instead of a dictionary this can also be a Q object for more complex queries."

Further discussion:

17:00 < chars> jacobkm: The only known case that broke the original .distinct() 
               solution was in Postgres. So maybe if #6422 gets accepted, we 
               could test for the distinct_on_fields feature and then distinct 
               on the pk, which is unique by definition.
17:00 < jacobkm> chars: see now *that* makes me a lot happier.
17:00 < chars> And fallback to the vanilla .distinct() if the backend doesn't 
               support it.

That's #6422.

[Chris Wesseling]

DISTINCT is just a special GROUP BY...

So an empty .annotate() does the trick too, since it groups by the pk. And the DBMS should by definition be able to compare pk's.
I'll try to put up a patch tonight.

[Chris Wesseling]

Replying to charstring:

DISTINCT is just a special GROUP BY...

So an empty .annotate() does the trick too, since it groups by the pk. And the DBMS should by definition be able to compare pk's.
I'll try to put up a patch tonight.

Well, that was a long night. ;)
I got implemented the .annotate() solution in here ​https://github.com/CharString/django/tree/ticket-11707

Is the PointField mentioned in 12 the same as the one that now lives in django.contrib.gis?

[Tim Graham]

I think the PointField in comment 12 is a custom field that's different from the one in contrib.gis. It's difficult for me to tell from the comments what the issue was. In any case, I'm going to mark this as "Patch needs improvement" since it appears it needs additional tests.

[Chris Wesseling]

Replying to charstring:

Is the PointField mentioned in 12 the same as the one that now lives in django.contrib.gis?

No, it isn't. I've installed postgis for this bug. postgis points *can* be tested on equality.. the PointField in 12 uses the builtin postgres point type, *not* the postgis point type that django.crontib.gis does.

[Crowley Shaita]

The solution is here ​https://github.com/django/django/pull/13315

[Carlton Gibson]

Unchecking Patch needs improvement so that this goes back into the review queue.

[Mariusz Felisiak]

Patch is not merged.

[Mariusz Felisiak]

Crowley, please stop changing a triage stage without any reason.

[Crowley Shaita]

Replying to felixxm:

Crowley, please stop changing a triage stage without any reason.

Sorry, I thought that's how I would get it to be reviewed.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 556fa4bb:

Fixed #1891, Fixed #11707 -- Prevented duplicates with limit_choices_to on multi-value relations.