How do you order by a custom model method that has no attribute in SQL?

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..

ruby-on-rails

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