extra row_number does not work

[Ye Xu]

Hi,

I have two models, as follow,
class Feed(models.Model):
    user=ForeignKey(...)

class Comment(models.Model):
    feed=ForeignKey(Feed, ...)
    comment_text=TextField(...)

I want get last two comments for each feed, the statement is:
Comment.objects.all().extr(select={'row_num': 'row_number() over (partition by feed_id order by id desc', where=['row_num<3']}

After run, Django told me these error:
....

Django Version:	1.8.2
Exception Type:	ProgrammingError
Exception Value:	column "row_num" does not exist LINE 1: ...nt"."create_date" FROM "footprint_comment" WHERE (row_num < ...

....

But remove where parameter, it works.
Comment.objects.all().extr(select={'row_num': 'row_number() over (partition by feed_id order by id desc'}