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dcramer / django-ratings

Pluggable rating fields in Django.
BSD 2-Clause "Simplified" License
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Not setting weight causes division by zero when calculating average (Postgres) #10

Open TimFletcher opened 14 years ago

TimFletcher commented 14 years ago

When I'm developing locally, I can leave the weight as default (0) and not get any errors when calculating the average rating using qs.extra(). However on a production server with Postgres this causes a 'Division by Zero' database error. I think this only happens when rating_votes+Recipe.rating.range is zero. I worked around this by setting the weight to 1.

tmpethick commented 13 years ago

Great! Thanks for the advise - it saved me from a lot of trouble! I couldn't find a easy solution to the problem and didn't think of looking at something as obvious as the weight.

rudedogg commented 11 years ago

Unfortunately if you have existing ratings before you notice/fix this issue, setting weight=1 will alter the ordering of ratings, in a way that for me made my sorting appear broken.

The other option is to use a case statement in the extra query.

Here is my code, I'm not getting the % of approval like the documentation example, this just orders by the avg rating expressed as a float. This allows you to not use the weight as a fix and keep your existing ratings. You'll need to use a template tag/filter to format the avg_rating for display.

    qs = qs.extra(select={'avg_rating': 'case when rating_votes = 0 then 0 else ((rating_score)/(rating_votes+%d)) end' % (MyModel.rating.weight)})

I think the example in the documentation would be better with the case statement added.