Django ORM如何在查询中引用另一行？ [英] Django ORM how to reference to another row in query?

问题描述

我有下表'book'：

I have the following table 'book':

id     shelf   position (on shelf)
 1        2        3
 2        1        1
 3        1        2
 4        2        2
 5        2        1

我需要将book4移到书架1的末尾，因此我需要将
book4的位置设置为[max（position），架子= 1] + 1
并在其中放置position = [max（position）where rack = 2]到book4的初始位置（以缩小差距）

I need to move book4 to the end of shelf 1, therefore I need to set book4 position to [max(position) where shelf=1] + 1 and position where position=[max(position) where shelf=2] to book4 initial position (to close the gap)

我尝试了这个，但是Max似乎没有用方式...

I tried this, but Max doesn't seem to work this way...

Book.objects.filter(shelf=2).annotate(max_position_on_shelf=Max('position'))\
                       .filter(position=F('max_position_on_shelf'))\
                       .update(position=F('max_position_on_shelf') + 1)

推荐答案

更简单：

from django.db.models import Max

class Book(models.Model):
    ....

    def move(self, shelf_num):
        Book.objects.filter(shelf=self.shelf).order_by('-position')[:1].update(position=self.position)
        self.position = Book.objects.filter(shelf=shelf_num).aggregate(Max('position')) + 1
        self.save()

这篇关于Django ORM如何在查询中引用另一行？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！