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I have an existing activerecord array, gotten by the following:

@posts = Post.where(["created_at > ?", n_days.days.ago])

And I would like to order this array based on a value that needs to be calculated in real-time from multiple columns:

posts_array = posts.map { |p| [p.id, f(t, p.x,p.y)] }

This gives an array like [[1,20],[2,11],[3,14], ...], which I then sort based on the function value to give [[1,20],[3,14],[2,11],...], and then ultimately get the order_array that I want: [1,3,2,...].

What is the best way to then reorder the original @posts array based on the new order_array [1,3,2,...]? I don't want to touch the database again if possible, since the information is already contained in the @posts array.

Is there a straightforward way to do this?

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    Do you already have a solution? If yes, please edit your question and add it, to avoid someone just typing what you already did (for example, sort followed by map) Commented May 9, 2013 at 13:39
  • I do not already have a solution that reorders the existing @posts array (my primary goal), nor do I have a solution that does just 1 more call (e.g. Post.find([1,3,2,...]) but with order in tact). I would be happy with either solution, but extra points for one that does not touch the database again. Commented May 9, 2013 at 13:42
  • What about doing it in sql? You create a query that generates a "custom" column for each record (calculated in realtime), than you just use sort('custom_column DESC'), it will heavily improve performance and reduce rails ram usage Commented May 9, 2013 at 13:47
  • Would that column actually be a database column? If you can give an example it might be what I want. Ultimately I don't want to write data to a bunch of columns though, since that seems like it would increase DB load by quite a bit. Commented May 9, 2013 at 13:49

2 Answers 2

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If you don't need the posts_array array you could do

@posts = Post.where(["created_at > ?", n_days.days.ago])
@posts.sort_by! { |p| f(t, p.x, p.y) }
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Did you mean sort_by!? Or just @posts = Post.where(...).sort_by{...}?
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Getting order_array

The obvious answer is:

[[1,20],[2,11],[3,14]].sort{|a,b| b[1] <=> a[1]}.map{|x| x[0]}
 => [1, 3, 2]

Not sure if someone can come up with something smarter?

On ruby 2.0, you can use lazy to avoid expand the array after the sort (and gain some performance)

[[1,20],[2,11],[3,14]].lazy.sort{|a,b| b[1] <=> a[1]}.map{|x| x[0]}

Retrieving posts in the order

The simplest solution is

Post.find(order_array).sort{|a,b| order_array.index(a.id) <=> order_array.index(b.id)}

But would be better simply

order_array.map{|x| Post.find(x)}

Or to use your already loaded @post list:

 order_array.map{|x| @post.find{|y| y.id == x)}
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2 Comments

Sorry, maybe my question was unclear. I don't have a problem getting the order_array, I have a problem getting the actual posts, based on the order [1,3,2,...].
fotanus thanks for the answer. Your last line works beautifully but giving the solution to toro2k for succinctness and eliminating an extra step. Thanks for yours too!

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