试图将 SQL 的左连接转换为 Django 查询集？

Question

这是我的models.py文件

class Customer(models.Model):
    """All Customers details goes here"""

    name = models.CharField(max_length=255, null=False)
    firm_name = models.CharField(max_length=255, null=False)
    email = models.EmailField(null=False)
    phone_number = models.CharField(max_length=255, null=False)
    location = models.CharField(max_length=255,null=True)
    date_created = models.DateTimeField(auto_now_add=True)

    class Meta:
        """Meta definition for Customer."""

        verbose_name = 'Customer'
        verbose_name_plural = 'Customers'

    def __str__(self):
        """Unicode representation of Customer."""
        return self.name


class Order(models.Model):
    """All order details goes here.It has OneToMany relationship with  Customer"""
    STATUS = (
        ('CR', 'CR'),
        ('DR', 'DR'),
    )

    customer = models.ForeignKey(Customer, null=True, on_delete=models.SET_NULL)
    bill_name = models.CharField(max_length=255, null=False)
    payment_date=models.DateField(auto_now=False)
    status = models.CharField(max_length=255, choices=STATUS, null=False)
    amount = models.FloatField(max_length=255, null=False)
    date_created = models.DateTimeField(auto_now_add=True)
    description = models.TextField(null=True)

    class Meta:
        """Meta definition for Order."""

        verbose_name = 'Order'
        verbose_name_plural = 'Orders'

    def __str__(self):
        """Unicode representation of Order."""
        return self.bill_name

我只想访问客户的姓名和订单的所有字段，简而言之，我想在 Django 查询集中转换以下 SQL

select name,bill_name,status from accounts_customer left join accounts_order on accounts_customer.id = accounts_order.customer_id where accounts_order.status="DR";

Answer 1

您可以使用两种方式执行连接操作：第一种：通过使用 select_related 即 Order.objects.select_related('customer') 第二种：通过使用过滤器：即 Order.objects.filter(status__iexact="DR")

Answer 2

要在订单 object 上附加客户姓名，您可以使用带有F表达式的annotate 。 https://docs.djangoproject.com/en/3.0/ref/models/expressions/#using-f-with-annotations

orders = Order.objects.annotate(
    customer_name=F('customer__name')
).filter(status='DR')

for order in orders:
    print(order.customer_name)

---

如果您怀疑要访问更多客户属性，则可能需要select_related （稍微多一点 memory，更大的查询）。 Django ORM中的select_related和prefetch_related有什么区别？

orders = Order.objects.select_related('customer').filter(status='DR')

for order in orders:
    print(order.customer.name)