Two foreign key fields, exactly one is set to a value, the other null in django model with MySQL database

I have a django model with two foreign key fields, one that points to product, the other points to portfolio in the code below. Exatcly one of these should be set for each Lca record. I know I can do this with MySQL triggers, but I was wondering if there was a way to make this conditional save in django

class Lca(models.Model):
    product             = models.ForeignKey(product, null=True, blank=True)
    portfolio           = models.ForeignKey(portfolio, null=True, blank=True)
    carbon_price        = models.DecimalField(max_digits=10, decimal_places=2, blank=True, null=True)
    name                = models.CharField(max_length=255, blank=True)
    total_footprint     = models.IntegerField(blank=True, null=True)
    calculation_type    = models.CharField(max_length=9)
    total_cv            = models.DecimalField(max_digits=10, decimal_places=0, blank=True, null=True)
    source              = models.CharField(max_length=255, blank=True)
    date_from           = models.DateField()
    date_to             = models.DateField(blank=True, null=True)

    def __unicode__(self):
        return self.name
    # end __unicode__
# end