https://docs.djangoproject.com/en/1.6/topics/db/examples/many_to_one/
这个官方文档的这部分是不是有问题啊，
Reporter是one的一方，Article是many的一方。。。。。。
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Querying in the opposite direction:
>>> Reporter.objects.filter(article__pk=1)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article=1)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article=a)
[<Reporter: John Smith>]
>>> Reporter.objects.filter(article__headline__startswith='This')
[<Reporter: John Smith>, <Reporter: John Smith>, <Reporter: John Smith>]
>>> Reporter.objects.filter(article__headline__startswith='This').distinct()
[<Reporter: John Smith>]
