Question

Need a little help with a SQL / ActiveRecord query. Let’s say I have this:

Article < ActiveRecord::Base
  has_many :comments
end

Comment < ActiveRecord::Base
  belongs_to :article
end

Now I want to display a list of “Recently Discussed” articles – meaning I want to pull all articles and include the last comment that was added to each of them. Then I want to sort this list of articles by the created_at attribute of the comment.

I have watched the Railscast on include /joins – very good, but still a little stumped.

I think I want to use a named_scope, something to this effect:

Article < ActiveRecord::Base
  has_many :comments

  named_scope :recently_commented, :include => :comments, :conditions => {   some_way_to_limit_just_last_comment_added }, :order => "comments.created_at DESC"
end

Using MySQL, Rails 2.3.4, Ruby 1.8.7

Any suggestions? 🙂

Answer 1

You have two solutions for this.

1) You treat n recent as n last. Then you don’t need anything fancy:

Article < ActiveRecord::Base   
  has_many :comments

  named_scope :recently_commented, :include => :comments, 
                :order => "comments.created_at DESC",
                :limit => 100 
end

Article.recently_commented # will return last 100 comments

2) You treat recent as in last x duration.

For the sake of clarity let’s define recent as anything added in last 2 hours.

Article < ActiveRecord::Base   
  has_many :comments

  named_scope :recently_commented, lambda { { 
                    :include => :comments, 
                    :conditions => ["comments.created_at >= ?", 2.hours.ago]
                    :order => "comments.created_at DESC",
                    :limit => 100 }} 

end

Article.recently_commented # will return last 100 comments in 2 hours

Note Code above will eager load the comments associated with each selected article.
Use :joins instead of :include if you don’t need eager loading.