python - 无法访问 annotate() 中的 to_attr 属性

Question

我有以下代码:

queue = Queue_item.objects.prefetch_related(Prefetch('queue_item_id', 
queryset=Platform.objects.all().order_by('-time'), to_attr='platforms'))
queue = queue.annotate(first_time=Min('platforms__time')).order_by('first_time')

当我运行它时，我收到一条错误消息，指出在 queue.annotate 中，它找不到平台，即使我之前在 prefetch_related 中正确设置了它也是如此。

我错过了什么？

[编辑]

#models.py

import django
from django.db import models

class Queue_item(models.Model):
caption = models.TextField(max_length=1000)
path_to_picture = models.CharField(max_length=255)

def create(self):
    self.save()

def __str__(self):
    return self.caption.split(' ')[0]

class Platform(models.Model):

queue_item_id = models.ForeignKey(
    Queue_item,
    on_delete=models.CASCADE
)

time = models.DateTimeField(default=django.utils.timezone.now)

INSTAGRAM = 'IG'
FACEBOOK = 'FB'
TWITTER = 'TW'

PLATFORM_CHOICES = (
    (INSTAGRAM, 'Instagram'),
    (FACEBOOK, 'Facebook'),
    (TWITTER, 'Twitter'),
)

platform = models.CharField(
    max_length=2,
    choices=PLATFORM_CHOICES,
    default=INSTAGRAM
)

谢谢!

Answer 1

嗯，反向关系名称是 django 默认使用的:platform(模型名称)，如下所示:

class Platform(models.Model):
    queue_item_id = models.ForeignKey(
        Queue_item,
        on_delete=models.CASCADE
    )

我们可以使用反向名称来执行查询，然后您尝试使用不正确的 s 访问它。

...<s>to_attr='platforms')</s>...
queue.annotate(first_time=Min('<s>platforms__time</s>'))

正确的是:

<b>....to_attr='platform')...</b>
queue.annotate(first_time=Min('<b>platform__time</b>'))

或

如果你想将 to_attr='platforms' & platforms__time 与 s 一起使用，你可以更改反向名称:

queue_item_id = models.ForeignKey(
    Queue_item,<b>reverse_name='platforms'</b>,
    on_delete=models.CASCADE
)

Since queue_item_id is a foreign key, no need to end the field name with _id. Just queue_item will be OK, Django has already create a similar for you like queue_item_id