Django中优化嵌套外键查询：告别N+1问题

from django.db import models

class A(models.Model):
    field1 = models.CharField(max_length=100)
    field2 = models.IntegerField()

    def __str__(self):
        return f"A-{self.id}"

class B(models.Model):
    field3 = models.CharField(max_length=100)
    field_a = models.ForeignKey(A, on_delete=models.CASCADE)

    def __str__(self):
        return f"B-{self.id}"

class C(models.Model):
    field4 = models.CharField(max_length=100)
    field5 = models.IntegerField()
    field_b = models.ForeignKey(B, on_delete=models.CASCADE)

    def __str__(self):
        return f"C-{self.id}"

    @property
    def nested_field(self):
        # 这种访问方式会导致额外的SQL查询
        return self.field_b.field_a

# 假设我们想访问 C -> B -> A
queryset = C.objects.select_related('field_b__field_a')
obj = queryset.first()

# 此时访问 nested_field 不会触发额外的SQL查询
print(obj.nested_field)
print(obj.field_b.field_a.field1)

from django.db.models import F

# 获取 C 对象的第一个关联 A 对象的 field1 字段
queryset = C.objects.annotate(nested_a_field1=F('field_b__field_a__field1'))
obj = queryset.first()

# nested_a_field1 现在是 C 对象的一个属性
print(obj.nested_a_field1) # 不会触发额外查询

from django.db.models import Manager, Model, F

class ModelCManager(Manager):
    def get_queryset(self):
        # 默认在查询 C 对象时就预加载 A 的 field1
        return (
            super().get_queryset()
            .annotate(a_field1=F('field_b__field_a__field1'))
        )

class C(Model):
    field4 = models.CharField(max_length=100)
    field5 = models.IntegerField()
    field_b = models.ForeignKey(B, on_delete=models.CASCADE)

    objects = Manager() # 默认管理器
    with_nested_a = ModelCManager() # 带有预加载 field1 的管理器

# 使用自定义管理器
queryset = C.with_nested_a.all()
obj = queryset.first()
print(obj.a_field1) # 通过 with_nested_a 查询时，a_field1 已可用

from django.db.models import F, Model, QuerySet

class ModelCQuerySet(QuerySet):
    def annotate_a_fields(self):
        """为 C 对象添加关联 A 模型的 field1 和 field2 字段。"""
        return self.annotate(
            a_field_1=F('field_b__field_a__field1'),
            a_field_2=F('field_b__field_a__field2')
        )

    def annotate_b_fields(self):
        """为 C 对象添加关联 B 模型的 field3 字段。"""
        return self.annotate(
            b_field_3=F('field_b__field3')
        )

class C(Model):
    field4 = models.CharField(max_length=100)
    field5 = models.IntegerField()
    field_b = models.ForeignKey(B, on_delete=models.CASCADE)

    objects = ModelCQuerySet.as_manager() # 将自定义 QuerySet 注册为默认管理器

# 链式调用自定义查询方法
queryset = (
    C.objects
    .filter(field_b__field4='some_value') # 假设 B 有 field4
    .annotate_a_fields()
    .annotate_b_fields()
)
obj = queryset.first()
print(obj.a_field_1)
print(obj.b_field_3)