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I'm attempting to edit an existing comment (i.e. replace old comment with a new one). My comments app is django.contrib.comments.

new_comment = form.cleaned_data['comment']

#all of the comments for this particular review
comments = Comment.objects.for_model(Review).filter(object_pk=review_id)

print comments[0].comment
#'old comment'

comments[0].comment = new_comment

print comments[0].comment
#'old comment' is still printed

Why is the comment not being updated with the new comment ?

Thank you.

Edit: Calling comments[0].save() and then print comments[0].comment, still prints 'old comment'

Himerzi
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2 Answers2

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You need to save the value

comments = comments[0]    
comments.comment = new_comment
comments.save()
Rakesh
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  • `comments` is a QuerySet. QuerySets don't have a `save()` method. – Himerzi Sep 24 '12 at 12:07
  • And comments[0].save() doesn't work, saves the old comment, not the new one. – Himerzi Sep 24 '12 at 12:08
  • Yes, this works - though I can't see why it's fundamentally different to my method. Any idea as to why `comments[0].comment = new_comment` doesn't work? – Himerzi Sep 24 '12 at 12:31
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This isn't to do with comments specifically. It's simply that querysets are re-evaluated every time you slice. So the first comments[0], which you change, is not the same as the second one - the second one is fetched again from the database. This would work:

comments = Comment.objects.for_model(Review).filter(object_pk=review_id)
comment = comments[0]
comment.comment = new_comment

Now you can save or print comment as necessary.

Daniel Roseman
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  • Brilliant, thanks for explaining why my method doesn't work. I thought Django caches recently execute queries ? Though, I suppose that doesn't mean the queryset isn't evaluated every time I slice. – Himerzi Sep 24 '12 at 12:34
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    Querysets are tricky. If the full queryset was previously evaluated/iterated, then it acts like a list and slicing won't hit the db again. But in your case the qs isn't evaluated, so slicing causes Django to go to the db each time to just get the items you've requested. – Daniel Roseman Sep 24 '12 at 13:11