Django-mptt 获取每个类别内项目的累积计数

Question

我正在使用 django-mptt。

我正在尝试获取所有类别以及每个类别内的项目数。

但我收到此错误：

OperationalError at /items/
(1054, "Unknown column 'items_category.lft' in 'where clause'")

从我的 view.py

    categories = Category.objects.add_related_count(
        Category.objects.all(),  # Queryset
        Item,  # Related mobile
        'category',  # Name of the foreignkey field
        'count',  # Name of the property added to the collection
        cumulative=True)  # Cumulative or not.
        
    print(categories)

model.py

class Category(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    slug = models.SlugField(max_length=50, null=True, blank=True, )
    parent = TreeForeignKey('self', on_delete=models.CASCADE, null=True, blank=True, related_name='children')

    class MPTTMeta:
        order_insertion_by=['name']

    def save(self, *args, **kwargs):
        self.slug = slugify(self.name)
        super(Category, self).save(*args, **kwargs)
    
    class Meta:
        unique_together = ('slug', 'parent',)    
        verbose_name_plural = "categories"   

class Item(models.Model):
    category = TreeForeignKey(Category, on_delete=models.SET_NULL, blank=True, null=True) 
    name = models.CharField(max_length=250 )

使用 print(categories.query) 生成的查询是：

SELECT `items_category`.`id`, `items_category`.`name`, `items_category`.`slug`, `items_category`.`parent_id`, `items_category`.`lft`, `items_category`.`rght`, `items_category`.`tree_id`, `items_category`.`level`, (SELECT count(*) FROM (SELECT U0.`id` FROM `items_item` U0 INNER JOIN `items_category` U1 ON (U0.`category_id` = U1.`id`) WHERE (U1.`lft` >= `items_category`.`lft` AND U1.`lft` <= `items_category`.`rght` AND U1.`tree_id` = `items_category`.`tree_id`)) _count) AS `count` FROM `items_category` ORDER BY `items_category`.`tree_id` ASC, `items_category`.`lft` ASC

我尝试修复查询生成器，直接在我的数据库上尝试它，似乎查询是正确的，但是语法非常错误。

正确且有效的查询是我刚刚删除了 `.

SELECT items_category.id, items_category.name, items_category.slug, items_category.parent_id, items_category.lft, items_category.rght, items_category.tree_id, items_category.level, 
    (
        SELECT count(*) 
            FROM (
                SELECT U0.id 
                FROM items_item U0 
                INNER JOIN items_category U1 ON ( U0.category_id = U1.id) 
                WHERE (U1.lft >= items_category.lft 
                       AND U1.lft <= items_category.rght 
                       AND U1.tree_id = items_category.tree_id
                )
            ) _count
    ) AS count 
                       FROM items_category 
ORDER BY items_category.tree_id 
    ASC, items_category.lft ASC