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         


        

Answer 1

As of Rails 4, you would do:

class Article < ActiveRecord::Base 
  has_many :comments, -> { order(created_at: :desc) }
end 
class Comment < ActiveRecord::Base 
  belongs_to :article 
end

For a has_many :through relationship the argument order matters (it has to be second):

class Article
  has_many :comments, -> { order('postables.sort' :desc) }, 
           :through => :postable
end

If you will always want to access comments in the same order no matter the context you could also do this via default_scope within Comment like:

class Comment < ActiveRecord::Base 
  belongs_to :article 
  default_scope { order(created_at: :desc) }
end

However this can be problematic for the reasons discussed in this question.

Before Rails 4 you could specify order as a key on the relationship, like:

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

As Jim mentioned you can also use sort_by after you have fetched results although in any result sets of size this will be significantly slower (and use a lot more memory) than doing your ordering through SQL/ActiveRecord.

If you are doing something where adding a default order is cumbersome for some reason or you want to override your default in certain cases, it is trivial to specify it in the fetching action itself:

sorted = article.comments.order('created_at').all