How to allow only one True value for a BooleanField ForeignKey to be accepted?

Question

Django

There is this model:

class ProductImage(models.Model):
    product = models.ForeignKey(_(u"Продукт"), Product, related_name='images')
    image = models.ImageField(_(u"Изображение"), upload_to="product_images/")
    is_main = models.BooleanField()

How can I make sure that among all ProductImages that refer to Product, there can only be 0 or 1 can take the value is_main=True?