Schema design for B2B price lists with per-customer prices and validity dates

# ======================================================================
# Q: Design the schema for B2B price lists (a different price per customer, with
# validity dates).
#
# A normalized schema, constraints in the database, and an index for the
# frequent lookup "price of product X for customer Y today". To prevent
# overlapping periods: the PostgreSQL exclusion constraint with daterange
# (ExclusionConstraint in django.contrib.postgres). The price charged is stored
# in OrderItem, because the price list changes.
# ======================================================================

class PriceList(models.Model):
    name = models.CharField(max_length=100)

class PriceListEntry(models.Model):
    price_list = models.ForeignKey(PriceList, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    price = models.DecimalField(max_digits=10, decimal_places=2)
    valid_from = models.DateField()
    valid_to = models.DateField(null=True, blank=True)

    class Meta:
        indexes = [models.Index(fields=['price_list', 'product', '-valid_from'])]
        constraints = [models.CheckConstraint(
            condition=Q(valid_to__isnull=True) | Q(valid_to__gte=F('valid_from')),
            name='pricelist_valid_range')]

class Customer(models.Model):
    price_list = models.ForeignKey(PriceList, null=True, on_delete=models.SET_NULL)