问题
So, I have the following scope in my Photo model:
scope :best, order(:average_rating.desc)
The only problem is, the ratings were added to the model after the fact, so the production app has a lot of records where average_rating is nil. When I call this scope it returns all the nils first -- in fact it should be the opposite, nils should be last ( they are photos which have not yet been rated ).
How can I sort nils to the end of this scope?
回答1:
I'm a bit late to the game but this just came up again and the solution really isn't that difficult.
A portable solution is to use a CASE to turn NULLs into something that will go to the end. If your ratings are non-negative, then -1 is a good choice:
order('case when average_rating is null then -1 else average_rating end desc')
Using COALESCE instead will work in many databases and is a lot less noisy than a CASE:
order('coalesce(average_rating, -1) desc')
If you wanted an ASC sort then the above approaches would put NULLs at the beginning. If you wanted them at the end then you'd use something bigger than your highest rating instead of -1. For example, if you rated things from one to five, you could use 11 to replace NULLs:
order('case when average_rating is null then 11 else average_rating end asc')
order('coalesce(average_rating, 11) asc')
Most people would use 10 but sometimes you need to get a little bit louder so ours go to 11.
You can't really depend on the database putting NULLs at the beginning or end so it is best to be explicit even if you're just reminding your future-self that you've handled the NULL case already.
回答2:
Try this :)
scope :best, order("average_rating DESC NULLS LAST")
回答3:
I know this was awhile ago but wouldn't this work across a few
order("ISNULL(average_rating) DESC")
回答4:
while the coalesce SQL method is great, if your database does not support this, here is a method that should be supported along all databaseses:
order("-average_rating DESC")
This will order your records and put the NULL to the end.
回答5:
Well, I never found an approach that would work across DB drivers, so I just forced the value of all the records that were previously nil to be zero. That solved it for me, although it was a bit brutish. When the timer runs out I'll accept this since it is the solution I used, but if anyone revisits this and wants to provide an alternative answer in the future I'll be happy to accept a different answer later on.
来源：https://stackoverflow.com/questions/5520628/rails-sort-nils-to-the-end-of-a-scope