Django外键赋值

classArticle(models.Model): title = models.CharField(max_length=1024, default='') ... def__str__(self): return 'Article pk:%d %s' % (self.pk, self.title[:30]) classArticleContent(models.Model): article = cached_fields.OneToOneField(Article) ...

写代码的的时候，发现了一个很奇怪的现象，当我给一个instance的外键（以_id结尾）赋值（数字）的时候 ，这个外键对应的instance的值并不会改变。

In [44]: ac = ArticleContent.objects.get(article_id=14269) In [45]: ac.article_id Out[45]: 14269 In [46]: ac.article_id = 14266 In [47]: ac.save() In [48]: ac.article Out[48]: <Article: Article pk:14266 EC: Russia, Ukraine to Meet in> In [49]: ac.article.pk Out[49]: 14266

如上面的代码所示，为了找到答案，我翻了一下Django的源码：

django/db/models/fields/related_descriptors.py 145 def__get__(self, instance, cls=None): 146 """ 147 Get the related instance through the forward relation. 148 149 With the example above, when getting ``child.parent``: 150 151 - ``self`` is the descriptor managing the ``parent`` attribute 152 - ``instance`` is the ``child`` instance 153 - ``cls`` is the ``Child`` class (we don't need it) 154 """ 155 if instance is None: 156 return self 157 158 # The related instance is loaded from the database and then cached in 159 # the attribute defined in self.cache_name. It can also be pre-cached 160 # by the reverse accessor (ReverseOneToOneDescriptor). 161 try: 162 rel_obj = getattr(instance, self.cache_name) 163 except AttributeError: 164 val = self.field.get_local_related_value(instance) 165 if None in val: 166 rel_obj = None 167 else: 168 qs = self.get_queryset(instance=instance) 169 qs = qs.filter(self.field.get_reverse_related_filter(instance)) 170 # Assuming the database enforces foreign keys, this won't fail. 171 rel_obj = qs.get() 172 # If this is a one-to-one relation, set the reverse accessor 173 # cache on the related object to the current instance to avoid 174 # an extra SQL query if it's accessed later on. 175 if not self.field.remote_field.multiple: 176 setattr(rel_obj, self.field.remote_field.get_cache_name(), instance) 177 setattr(instance, self.cache_name, rel_obj) 178 179 if rel_obj is None and not self.field.null: 180 raise self.RelatedObjectDoesNotExist( 181 "%s has no %s." % (self.field.model.__name__, self.field.name) 182 ) 183 else: 184 return rel_obj

注释得非常到位，当我们请求 ac.article 的时候，会先去检查对应的cache（在这里是 _article_cache ，感兴趣可以去看 cache_name 的生成规则，就是外键名前面加下划线，后面加 cache ）存不存在，如果不存在那么就进行数据库请求，请求完之后会保存到 cache 中。

我们再看看 __set__ ，代码太长就不贴了（就在 __get__ 方法下面）。除了给外键字段（ article ）赋值外，还会将pk字段（ article_id ，是 lh_field.attname 的值）设置为 None ，这样下次请求的时候就能拿到正确的值。

以上都是 ForeignKey 的Magic，而当我们给 article_id 赋值的时候，只是在给一个普通的attribute赋值而已，没有任何magic，不会清理对应外键的cache，这时候拿到的instance仍然是cache中 原来的那个instance 。