反向关联报错AttributeError: ‘ForeignKey‘ object has no attribute ‘rel‘从django源码找答案~

AttributeError: ‘ForeignKey’ object has no attribute ‘rel’

因为django版本不一样
旧版本可以调用ForeignKey.rel.model.objects.all()
新版本中
这里的rel属性被设置为ReverseManyToOneDescriptor的实例才能使用在源码中有如下信息：

class ReverseManyToOneDescriptor:
    """
    Accessor to the related objects manager on the reverse side of a
    many-to-one relation.

    In the example::

        class Child(Model):
            parent = ForeignKey(Parent, related_name='children')

    ``Parent.children`` is a ``ReverseManyToOneDescriptor`` instance.
	"""
	def __init__(self, rel):
        self.rel = rel

在这里设置了反向多对一的时候只有这个类的实例才能用rel，而ForeignObject想用rel属性只能根据下边的方法去寻找。

from django.db import models

在django中建表用到的外键对象（models.ForeignKey），先看一下ForeignKey的__mro__(方法解析顺序)

models.ForeignKey.__mro__
(<class 'django.db.models.fields.related.ForeignKey'>, <class 'django.db.models.fields.related.ForeignObject'>, <class 'django.db.models.fields.related.RelatedField'>, <class 'django.db.models.fields.mixins.FieldCacheMixin'>, <class 'django.db.models.fields.Field'>, <class 'django.db.models.query_utils.RegisterLookupMixin'>, <class 'object'>)

先去第一个父类中找

class ForeignKey(ForeignObject):
 	def __init__(self, to, on_delete, related_name=None, related_query_name=None,
                 limit_choices_to=None, parent_link=False, to_field=None,
                 db_constraint=True, **kwargs):
    	...
	    kwargs['rel'] = self.rel_class(
	            self, to, to_field,
	            related_name=related_name,
	            related_query_name=related_query_name,
	            limit_choices_to=limit_choices_to,
	            parent_link=parent_link,
	            on_delete=on_delete,
	        )
       	super().__init__(to, on_delete, from_fields=['self'], to_fields=[to_field], **kwargs)

在这里rel被丢给父级的初始化方法，进去继续寻找

class ForeignObject(RelatedField):
	def __init__(self, to, on_delete, from_fields, to_fields, rel=None, related_name=None,
	                 related_query_name=None, limit_choices_to=None, parent_link=False,
	                 swappable=True, **kwargs):
	  	...
       if rel is None:
         rel = self.rel_class(
             self, to,
             related_name=related_name,
             related_query_name=related_query_name,
             limit_choices_to=limit_choices_to,
             parent_link=parent_link,
             on_delete=on_delete,
         )

        super().__init__(rel=rel, **kwargs)

在ForeignObject中，因为kwargs中设置了rel所以这里的if rel is None不会被执行，
rel没有改变，继续传递给父级的初始化方法

class Field(RegisterLookupMixin):
    def __init__(self, verbose_name=None, name=None, primary_key=False,
                 max_length=None, unique=False, blank=False, null=False,
                 db_index=False, rel=None, default=NOT_PROVIDED, editable=True,
                 serialize=True, unique_for_date=None, unique_for_month=None,
                 unique_for_year=None, choices=None, help_text='', db_column=None,
                 db_tablespace=None, auto_created=False, validators=(),
                 error_messages=None):
        ...
        self.remote_field = rel

终于被赋值了 所以在这里想用的话就要用：

ForeignKey.remote_field.model.objects.all()

本文地址：https://blog.csdn.net/weixin_43832745/article/details/107350861