Proposed opt_group argument for ModelChoiceField and ModelMultipleChoiceField

[Héctor Urbina]

Hello,

I've just implemented this and I thought It could well be incorporated into Django itself; I guess it's a fairly common feature that one may need on any project.

What I propose is to add a opt_group argument to ModelChoiceField and ModelMultipleChoiceField; which indicates the item's field whose value is used to group the choices. It should be used in conjunction with a queryset which is (primarily) sorted by the same field.

Let me show with an example:

class Category(models.Model):
    name = models.CharField(max_length=20)

class Item(models.Model):
    name = models.CharField(max_length=20)
    category = models.ForeignKey(Category)

And in some form's initialization process

field = ModelChoiceField(queryset=Item.objects.order_by('category__name', 'name'), opt_group='category')

field.choices will dynamically collect choices into named groups as a 2-tuple, which the underlying widget should present using optgroup HTML elements.