use custom methods for sorting in rails .sort!

Question

I have am serving up a bunch of objects into a status feed and want to sort them based off their latest activity. If I were just sorting off of the created_at attribute of each object I would use the code:

    articles = Article.all
    documents = Document.all
    @feeds = (articles + documents).sort! { |b, a| a.created_at <=> b.created_at }

But let's say that people can post in documents such that the latest activity in a document is the created_at attribute in the latest post. Then I would like to sort as such:

 @feeds = (articles + documents).sort! { |b, a| a.latest_activity <=> b.latest_activity }

And I would define latest_activity in the models:

In Document.rb

 def latest_activity
      self.posts.last.created_at
 end

in Article.rb

 def latest_activity
     self.created_at
 end

This doesn't work.. the local server serves the following error:

 undefined method `created_at' for nil:NilClass

How can I accomplish the sorting desired here?

Shot in the dark-- Documents with no posts results in self.posts.last being nil? — Platinum Azure
– Platinum Azure, Commented Sep 12, 2011 at 19:58
hmmmm Platinum Azure, good job - totally forgot to add a check for documents with no posts!!! — jay
– jay, Commented Sep 13, 2011 at 16:04

Jesse Wolgamott · Accepted Answer · 2011-09-12 20:00:57Z

1

I would normally use the "updated_at" rather than created_at. Then, in your Post class:

class Post < ActiveRecord::Base
  belongs_to :document, :touch => true
end

This means that when a post is saved, it will update the document's updated_at column.

Then, to simplify:

articles = Article.all
documents = Document.all
@feeds = (articles + documents).sort_by(&:updated_at)

answered Sep 12, 2011 at 20:00

Jesse Wolgamott

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