Django and fetching of foreign keys on attribute access.

Today I noticed something funny about how, and when, Django automatically fetches related objects.

With this simple model;

from django.db import models

class Blog(models.Model):
    title = models.CharField(max_length=128)

   def __unicode__(self):
       return self.title

class Article(models.Model):
     body = models.TextField()
     blog = models.ForeignKey(Blog)

If you fetch the article and access it's blog.id field, the related object is automatically
fetched, as we can see by investigating the instances _blog_cache field:

>>> blog = Blog.objects.create(title="My Blog")
>>> article = Article.objects.create(body="The quick brown fox...", blog=blog)
>>> article.blog.id
1
>>> article._blog_cache
<Blog: My Blog>

However, if you have no interest in fetching
the related object, there seems to be a workaround: the blog_id field, just check it out;

>>> article = Article.objects.all()[0]
>>> article.blog_id
1
>>> article._blog_cache
AttributeError: 'Article' object has no attribute '_blog_cache'