Django map加入原始请求

Question

我有以下代码：

class Invoice(models.Model):
    customer      = models.ForeignKey(User, blank=True, null=True)
    customer_name = models.CharField(max_length=50, blank=True, null=True)
    email         = models.CharField(max_length=100, blank=True, null=True)

#----------------------------------

invoices = Invoice.objects.raw("""
    SELECT
        `invoices`.`id`,
        `invoices`.`customer_id`,
        `invoices`.`customer_name`,
        `invoices`.`email` AS `inv_email`,
        `auth_user`.`username`,
        `auth_user`.`email` AS `auth_email`,
         COUNT('customer_id') AS `buy_count`

    FROM `invoices`
        LEFT JOIN `auth_user` ON `auth_user`.id = `invoices`.customer_id

    GROUP BY `customer_id`, `invoices`.`email`
""", translations={'inv_email': 'email', 'auth_email': 'customer.email'})

但是，当我写invoices[i].customer时，Django会为每个客户提出SQL请求。有办法在原始请求中将JOIN映射到Django模型吗？或者这个SQL请求可以使用纯Django ORM来实现？