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 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 2

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