Django QuerySet 外部キー制約無しで結合する

内部結合(直積した結果からユーザーIDが等しいもの)

Customer.objects.extra(
  tables=['django_user'],
  where=['customers.created_user_id=django_user.id']
).extra(select={'username': "django_user.last_name+django_user.first_name"})

SELECT (django_user.last_name+django_user.first_name) AS `username`,
       `customers`.`id`,
       `customers`.`created_at`,
       `customers`.`created_user_id`
FROM `customers`,
     `django_user`
WHERE (customers.created_user_id=django_user.id)
ORDER BY `customers`.`id` ASC

外部結合

https://stackoverflow.com/questions/21271835/left-join-django-orm
https://github.com/django/django/blob/master/django/db/models/sql/datastructures.py#L26-L139

joinField = ForeignKey(
    to=User,
    on_delete=lambda: x,
)

joinField.opts = Options(Customer._meta)
joinField.opts.model = Customer
joinField.get_joining_columns = lambda: (("created_user_id", "id"),)

j = Join(
    User._meta.db_table,
    Customer._meta.db_table,
    'T1',
    "LEFT JOIN",
    joinField,
    True
)

q = Customer.objects.values()
q = q.extra(select={'username': "django_user.last_name+django_user.first_name"})

q.query.join(j)

print(q.query)