from django.db import models


class Address(models.Model):
    company = models.CharField(max_length=1)
    customer_id = models.IntegerField()

    class Meta:
        unique_together = [
            ('company', 'customer_id'),
        ]


class Customer(models.Model):
    company = models.CharField(max_length=1)
    customer_id = models.IntegerField()
    address = models.ForeignObject(
        Address, models.CASCADE, null=True,
        # order mismatches the Contact ForeignObject.
        from_fields=['company', 'customer_id'],
        to_fields=['company', 'customer_id'],
    )

    class Meta:
        unique_together = [
            ('company', 'customer_id'),
        ]


class Contact(models.Model):
    company_code = models.CharField(max_length=1)
    customer_code = models.IntegerField()
    customer = models.ForeignObject(
        Customer, models.CASCADE, related_name='contacts',
        to_fields=['customer_id', 'company'],
        from_fields=['customer_code', 'company_code'],
    )