Customize select widget in a foreignkey field

Question

I have the following models:

class RegionCategory(models.Model):
    """Categories of regions"""

    symbol = models.CharField(max_length=15)
    description = models.CharField(max_length=50)

class Region(models.Model):
    """Region model"""

    region = models.CharField(max_length=30)
    category = models.ForeignKey(RegionCategory)

class md_orderlog(models.Model):
    """Order log model"""

    order_region = models.ForeignKey(Region)

Nothing uncommon, just a model with a foreign key to another model.

I have a ModelForm also:

class md_orderlogForm(forms.ModelForm):
    """Model form of md_orderlog model"""

    def __init__(self, *args, **kwargs):
        super(md_orderlogForm, self).__init__(*args, **kwargs)

        self.fields['order_region'] = forms.ModelChoiceField(
              label=("Project Region"), widget=forms.Select,   
              queryset=Region.objects.all())

What I want and I don't know how to do is to customize the order_region field in the form so the select show the Region items divided by the category, something like this:

Category1
    Region1
    Region2
    Region3 
Category2
    Region4
    Region5
    Region6

Any ideas?, thanks!