'distinct':True no longer usable in limit_choices_to

Sorry, but sqlite does not enforce foreign keys at all, see http://www.sqlite.org/omitted.html

quoting:

SQL Features That SQLite Does Not Implement

Rather than try to list all the features of SQL92 that SQLite does support, it is much easier to list those that it does not. Unsupported features of SQL92 are shown below. The order of this list gives some hint as to when a feature might be added to SQLite. Those features near the top of the list are likely to be added in the near future. There are no immediate plans to add features near the bottom of the list.

FOREIGN KEY constraints FOREIGN KEY constraints are parsed but are not enforced.

Sorry, the previous text was meant for a different bug. Please ignore it ...

I don't see how distinct=True makes any sense in the context of limit_choices_to. The result of limit_choices_to will have to include the primary key (that's the whole point of ForeignKey) so a SELECT DISTINCT will never return fewer rows than a plain old SELECT. I'm closing this for now. Please re-open it if you think I'm missing something.

Try this:

Create an Asset in admin (in the example models above) called proxy1.

Add both Socks Proxy and Web Proxy groups to it.

Then without distinct=True, when you go to create a Proxy object you see the proxy1 Asset twice in the dropdown in the admin. Using distinct=True in Pre-MR django reduced this to 1 proxy1 item in the dropdown.

Ahhh I see now. groups__name__in is causing a join between the Proxy table and the Asset <-> Group many to many table. The extra Group objects are making extra distinct rows.

Trying to shoehorn all the new manager methods into kwargs is not going to be pretty. What if limit_choices_to was deprecated in favor of passing a QuerySet as the choices argument?

For example:

PROXY_GROUPS = ('Terminal Server', 'Socks Proxy', 'Web Proxy')

class Group(models.Model):
    name = models.CharField(maxlength=32)

class Asset(models.Model):
    name = models.CharField(maxlength=32)
    groups = models.ManyToManyField(Group)

class Proxy(models.Model):
    asset = models.ForeignKey(Asset, choices=Asset.objects.filter('groups__name__in': PROXY_GROUPS).distinct())
    port = models.PositiveIntegerField()

Using a a QuerySet? is what I discussed with malcomt on #django but we didn't get too far with it. As long as it could handle the choices changing that would be great, ie. adding/removing assets from the PROXY_GROUPS. Would it only take a QuerySet? or any callable?

I had a "doh!" moment about this bug in the shower this morning: we don't need to make "distinct" something that is specified by the user. We can work out precisely when it is required and just Do The Right Thing(tm).

If the limit_choices_to attribute adds a ForeignKey? field to the query that does not have unique=True, then we should be using "select distinct". Otherwise, a simple "select" does the trick. There are some subtleties such as if we include a combination of fields that together comprise a "unique_together" block, then we don't need "distinct" any longer (because it is back to being unique by default).

Although the explanation looks complex, the code is probably not going to be too bad (although adding anything to django/db/models/query.py makes my head hurt). I'll look at writing a patch for this when I get some time (probably on the weekend, since it's queued up behind some other Django work).

Would it make sense to combine 'choices' and 'limit_choices_to' into one field option?

Replying to jkocherhans:

What if limit_choices_to was deprecated in favor of passing a QuerySet as the choices argument?

This would be cool.

Tagging for queryset refactor - maybe something that Malcolm wants to look at. I also like the idea of deprecating limit_choices_to and adding the option of passing a QuerySet? to choices

Patch against qs-rf

I have attached a patch which should allow you to pass either a callable or a QuerySet? to the choices parameter of a relational field, my one concern however is that it is not possible to use the limit/offset syntax with this because the QuerySet? is evaluated and that would cause the queryset to be evauated at load time, rather than each time the choices need to be generated.

On newforms-admin, you can use ModelAdmin.get_form to return a custom from with a custom model choice field that makes use of the queryset argument.

That solution only applied to admin. limit-choices-to is used for more than just the admin interface... it's useful wherever you extract the list of options for a ForeignKey.