Django自定义多对多关系下QuerySet过滤报错问题排查

问题背景 #

现有以下Django模型，Block和CustomCondition通过中间模型BlockCondition实现多对多关联，中间模型包含自定义字段choice：

from django.db import models


class CustomCondition(models.Model):
    name = models.CharField("custom condition name")

    class Meta:
        db_table = "custom_conditions"

    def __str__(self) -> str:
        return self.name


class BlockCondition(models.Model):
    block = models.ForeignKey(
        "Block",
        related_name="block_condition_as_block",
        on_delete=models.CASCADE,
    )
    custom_condition = models.ForeignKey(
        CustomCondition,
        related_name="block_condition_as_custom_condition",
        on_delete=models.CASCADE,
    )
    choice = models.CharField(
        "choice",
        choices=[
            ("NO_CONDITION", "No condition"),
            ("ACTIVATED", "Activated"),
            ("NOT_ACTIVATED", "Not activated"),
        ],
        default="NO_CONDITION",
    )

    class Meta:
        db_table = "block_conditions"

    def __str__(self) -> str:
        return self.custom_condition.name


class Block(models.Model):
    name = models.CharField("name", null=False)
    block_conditions = models.ManyToManyField(
        CustomCondition,
        through="BlockCondition",
        blank=True,
        default=None,
        related_name="blocks_as_block_condition",
    )

    class Meta:
        db_table = "blocks"

需求是遍历每个CustomCondition，筛选出对应条件处于ACTIVATED或NO_CONDITION状态的Block，执行以下代码时触发错误：

for c in CustomCondition.objects.all():
    if c.name:
        filtered_blocks = Block.objects.filter(
            block_conditions__custom_condition__name=c.name
        )
        filtered_blocks = filtered_blocks.filter(
            block_conditions__choice__in=["ACTIVATED", "NO_CONDITION"]
        )
        print(filtered_blocks)

报错信息：

django.core.exceptions.FieldError: Unsupported lookup 'custom_condition' for ForeignKey or join on the field not permitted.

错误原因 #

1. block_conditions是Block关联CustomCondition的多对多字段，直接访问block_conditions__custom_condition属于逻辑错误——block_conditions本身就是CustomCondition实例，不需要再通过custom_condition字段反向关联。
2. 要查询中间模型BlockCondition的choice字段，不能直接通过多对多字段block_conditions，必须通过中间模型的关联关系（即Block中block_condition_as_block这个related_name）来访问中间模型的字段。

修正后的代码 #

方式一：遍历每个CustomCondition查询（符合原需求逻辑） #

for c in CustomCondition.objects.all():
    if c.name:
        filtered_blocks = Block.objects.filter(
            # 通过中间模型的关联找到对应的CustomCondition
            block_condition_as_block__custom_condition=c,
            # 筛选中间模型的choice状态
            block_condition_as_block__choice__in=["ACTIVATED", "NO_CONDITION"]
        )
        print(filtered_blocks)

方式二：更高效的批量查询（避免循环查库） #

如果不需要单独处理每个CustomCondition，可以一次性查询所有符合条件的Block与CustomCondition的对应关系：

from django.db.models import Prefetch

# 预取符合条件的中间模型数据
prefetch = Prefetch(
    'block_condition_as_block',
    queryset=BlockCondition.objects.filter(
        choice__in=["ACTIVATED", "NO_CONDITION"]
    ),
    to_attr='valid_block_conditions'
)

# 查询所有带有有效条件的Block
blocks = Block.objects.prefetch_related(prefetch).filter(
    valid_block_conditions__isnull=False
)

# 遍历输出每个Block对应的有效CustomCondition
for block in blocks:
    print(f"Block: {block.name}")
    for bc in block.valid_block_conditions:
        print(f"  CustomCondition: {bc.custom_condition.name}, Choice: {bc.choice}")

内容的提问来源于stack exchange，提问作者bolino