关于Django外键赋值问题详解

本文主要给大家介绍关于Django外键赋值的相关内容，分享出来供大家参考学习，在开始之前，我们先来看一段代码：

class Article(models.Model):
 title = models.CharField(max_length=1024, default='')
 ...
 def __str__(self):
  return 'Article pk:%d %s' % (self.pk, self.title[:30])

class ArticleContent(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 
  def __get__(self, instance, cls=None):
   """
   Get the related instance through the forward relation.

   With the example above, when getting ``child.parent``:

   - ``self`` is the descriptor managing the ``parent`` attribute
   - ``instance`` is the ``child`` instance
   - ``cls`` is the ``Child`` class (we don't need it)
   """
   if instance is None:
    return self

   # The related instance is loaded from the database and then cached in
   # the attribute defined in self.cache_name. It can also be pre-cached
   # by the reverse accessor (ReverseOneToOneDescriptor).
   try:
    rel_obj = getattr(instance, self.cache_name)
   except AttributeError:
    val = self.field.get_local_related_value(instance)
    if None in val:
     rel_obj = None
    else:
     qs = self.get_queryset(instance=instance)
     qs = qs.filter(self.field.get_reverse_related_filter(instance))
     # Assuming the database enforces foreign keys, this won't fail.
     rel_obj = qs.get()
     # If this is a one-to-one relation, set the reverse accessor
     # cache on the related object to the current instance to avoid
     # an extra SQL query if it's accessed later on.
     if not self.field.remote_field.multiple:
      setattr(rel_obj, self.field.remote_field.get_cache_name(), instance)
    setattr(instance, self.cache_name, rel_obj)

   if rel_obj is None and not self.field.null:
    raise self.RelatedObjectDoesNotExist(
     "%s has no %s." % (self.field.model.__name__, self.field.name)
    )
   else:
    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。

总结