Previously I ordered my posts as this :
@posts = Post.find(:all, :order => "created_at DESC")
But now I want to replace created_at with a custom method I wrote in the Post model that gives a number as its result.
My guess:
@posts = Post.find(:all, :order => "custom_method DESC")
which fails..
Well, just Post.find(:all) would return an array of AR objects. So you could use Array.sort_by and pass it a block, and since those records are already fetched, you can access the virtual attribute inside the block that sort_by takes.
RDoc: Enumerable.sort_by
It fails because you are asking your db to do the sorting.
@posts = Post.all.sort {|a,b| a.custom_method <=> b.custom_method}
Note that this becomes non-trivial when you want to start paging results and no longer wish to fetch .all. Think about your design a bit before you go with this.
Just to expand on @Robbie's answer
Post.all.sort_by {|post| post.custom_method }.reverse