查找只有一个特定相关模型的模型

Question

考虑以下模型：

class Product(models.Model):
    name = models.CharField(max_length=...)

class Size(models.Model):
    name = models.CharField(max_length=...)
    products = models.ManyToManyField(Product, through=ProductXSize,
        related_name='sizes', related_query_name='size')

class ProductXSize(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE,
        related_name='productxsizes', related_query_name='productxsize')
    size = models.ForeignKey(Size, on_delete=models.CASCADE,
        related_name='productxsizes', related_query_name='productxsize')

我想要实现的是：

for p in Product.objects.filter(sizes=[Size.object.get(...)]):
    ...

也就是说，找到具有一种尺寸的产品，并且是特定尺寸的产品。

Answer 1

您需要用聚合值注释查询集：

https://docs.djangoproject.com/en/2.1/topics/db/aggregation/#filtering-on-annotations

Product.objects.annotate(size_cnt=Count('size'))\  # annotate
    .filter(size_cnt=1)\  # keep all that have only one size
    .filter(size=...).all()  # keep all with a certain size

查找只有一个特定相关模型的模型

Find models that have only one particular related model

django

django-models

many-to-many

django-orm