Django如何支持group by
users = User.objects.all()
users.query.group_by = ['col1']? #col1是数据库中的表的字段名
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http://stackoverflow.com/questions/327807/django-equivalent-for-count-and-group-by
Item.objects.values('category').annotate(Count('category'))
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http://docs.djangoproject.com/en/dev/topics/db/aggregation/
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