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

Question

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

Answer 1

You can override the model's save method:

def save(self, *args, **kwargs):
    if self.product and self.portfolio or not self.product and not self.portfolio:
        raise ValueError('Exactly one of [Lca.product, Lca.portfolio] must be set')

    super(Lca, self).save(*args, **kwargs)

Note that this method is not applicable to bulk_create.