Django form; getting secondary information

I'm developing a simple webshop like application for a website. The shop is for various clothing equpment, wich can be ordered in different sizes. I've set up the following structure:

SHOP_SIZE_TYPES = (
    ('Letter', 'letter'),
    ('Number', 'number'),
)


class ShopSize(models.Model):

    sizetype        = models.CharField('Sizetype',max_length = 100, choices=SHOP_SIZE_TYPES)
    size            = models.CharField('Size', max_length = 100)

    def __unicode__(self):
        return self.size


class ShopEquipment(models.Model):

    title = models.CharField("Navn", max_length = 100);
    image = models.ImageField();
    price = models.IntegerField("Pris")
    image   = models.ImageField(upload_to='shop_equipment');


    availible_sizes = models.ManyToManyField(ShopSize);

    def __unicode__(self):
        return self.title

When a visitor buys a item, the following is to be stored in the database:

class OrderItem(models.Model):
    order           = models.ForeignKey('Order')
    equipment       = models.ForeignKey('ShopEquipment')
    price           = models.IntegerField('Price at time of purchase')
    size            = models.CharField('Size', max_length = 100);

My trouble is when I want to present a form for each Equipment, simply containing a send-button and a Select-meny containing the availible sizes for that specific item. I can't figure out how to populate that select-menu, based on which equipment that specific form i for!

Is there a way to do this, or should I actually just render the form manually in the templa开发者_如何学Pythonte, using html and {% for %} loops?

Thanks!