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"],

    )