How do I automatically sort a has_many relationship in Rails?

Question

This seems like a really simple question but I haven't seen it answered anywhere.

In rails if you have:

class Article < ActiveRecord::Base 
  has_many :comments 
end 
class Comments < ActiveRecord::Base 
  belongs_to :article 
end

Why can't you order the comments with something like this:

@article.comments(:order=>"created_at DESC")

Named scope works if you need to reference it a lot and even people do stuff like this:

@article.comments.sort { |x,y| x.created_at <=> y.created_at }

But something tells me it should be simpler. What am I missing?

Answer 1

You can use ActiveRecord's find method to get your objects and sort them too.

  @article.comments.find(:all, :order => "created_at DESC")

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

Answer 2

You can specify the sort order for the bare collection with an option on has_many itself:

class Article < ActiveRecord::Base 
  has_many :comments, :order => 'created_at DESC'
end 
class Comment < ActiveRecord::Base 
  belongs_to :article 
end

Or, if you want a simple, non-database method of sorting, use sort_by:

article.comments.sort_by &:created_at

Collecting this with the ActiveRecord-added methods of ordering:

article.comments.find(:all, :order => 'created_at DESC')
article.comments.all(:order => 'created_at DESC')

Your mileage may vary: the performance characteristics of the above solutions will change wildly depending on how you're fetching data in the first place and which Ruby you're using to run your app.

Answer 3

If you are using Rails 2.3 and want to use the same default ordering for all collections of this object you can use default_scope to order your collection.

class Student < ActiveRecord::Base
  belongs_to :class

  default_scope :order => 'name'

end

Then if you call

@students = @class.students

They will be ordered as per your default_scope. TBH in a very general sense ordering is the only really good use of default scopes.