# How to ? Rails 3 ActiveRecord eager loading and AREL **URL:** \ **Category:** Rails\ **Created:** [August 5, 2010, 4:48pm UTC](https://www.ruby-forum.com/t/how-to-rails-3-activerecord-eager-loading-and-arel/192793 "2010-08-05T16:48:59Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![m-ryan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e0b2c6/32.png) [@m-ryan](https://www.ruby-forum.com/u/m-ryan)\ **Post date:** [August 5, 2010, 4:48pm UTC](https://www.ruby-forum.com/t/how-to-rails-3-activerecord-eager-loading-and-arel/192793/1 "2010-08-05T16:48:59Z")
Hello everyone, I would like to eager load scoped records to avoid queries executed in a loop (huge performance impact). The “scoped” part is what is giving me a hard time. I’m using Rails3 RC. Does anyone have any idea how I can do it? Here is a test case : we have an “Article” and a “Comment” Activerecord models, article has many comments comment belongs to article Comment has a scope defined like this (app/models/comment.rb) : * * * def recent where [“published\_at \>= ?”, 2.days.ago] end * * * In the ArticlesController (app/controllers/comments\_controller.rb) : * * * def index @articles = Article.includes(:comments) end * * * In the view (haml) (app/views/articles/index.html.haml): * * * - @articles.each do |article| = article.title - article.comments.each do |comment| = comment.body * * * this works fine and no query is executed when accessing the comments for each articles (“includes” call in the controller handled the eager loading). But if you try to display only the recent comments for each article : * * * - @articles.each do |article| = article.title - article.comments.recent.each do |recent\_comment| = recent\_comment.body * * * Now a query is executed for every article in order to fetch it’s recent articles. What can I write (presumably in the “include” call) to get the eager loading to work in this situation ? Cheers, M-Ryan ---
**Author:** ![m-ryan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e0b2c6/32.png) [@m-ryan](https://www.ruby-forum.com/u/m-ryan)\ **Post date:** [August 5, 2010, 7:30pm UTC](https://www.ruby-forum.com/t/how-to-rails-3-activerecord-eager-loading-and-arel/192793/2 "2010-08-05T19:30:59Z")
On Aug 5, 9:48 am, M- Ryan [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hello everyone, > > I would like to eager load scoped records to avoid queries executed in a > loop (huge performance impact). > > The “scoped” part is what is giving me a hard time. Can you define a scope at class level, instead of defining a method? scope :recent, where [“published\_at \>= ?”, 2.days.ago] Then when you later do article.comments.recent the join/include clauses should be chained together _first_, resulting in max one query per article. Jeff ---
**Author:** ![m-ryan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e0b2c6/32.png) [@m-ryan](https://www.ruby-forum.com/u/m-ryan)\ **Post date:** [August 6, 2010, 9:03am UTC](https://www.ruby-forum.com/t/how-to-rails-3-activerecord-eager-loading-and-arel/192793/3 "2010-08-06T09:03:32Z")
Thanks for your replay Jeff. Unfortunately, declaring my scope with the “scope” method instead of a regular method returning an ActiveRecord::Relation doesn’t solve my problem. (it also creates a new one since “2.days.ago” is only evaluated when the app loads the classes and not every time the method is called) I think the includes method should support relations as well as associations (since associations/scopes/relations are so well integrated with eachother in rails 3). That way the following : @articles = Article.includes(:comments =\> :recent) would build the relation in a way that recent comments are eagerly loaded (instead of complaining that comments have no association named “recent”) Cheers, M-Ryan ---
**Author:** ![m-ryan](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/e0b2c6/32.png) [@m-ryan](https://www.ruby-forum.com/u/m-ryan)\ **Post date:** [August 6, 2010, 8:40pm UTC](https://www.ruby-forum.com/t/how-to-rails-3-activerecord-eager-loading-and-arel/192793/4 "2010-08-06T20:40:46Z")
On Aug 6, 3:03 am, M- Ryan [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Unfortunately, declaring my scope with the “scope” method instead of a > regular method returning an ActiveRecord::Relation doesn’t solve my > problem. (it also creates a new one since “2.days.ago” is only evaluated > when the app loads the classes and not every time the method is called) He made a mistake. It should be: scope :recent, lambda { where [“published\_at \>= ?”, 2.days.ago] } > I think the includes method should support relations as well as > associations (since associations/scopes/relations are so well integrated > with eachother in rails 3). > That way the following : > > @articles = Article.includes(:comments =\> :recent) Ok, I see. You want to scope the eager-loaded objects. I guess this is a case where Datamapper’s identity map comes in handy. I haven’t tried this yet, but you might be able to put add scopes to the association itself. has\_many :recent\_comments, :class\_name =\> “Comment”, :conditions =\> … Hm, though that won’t do the lazy eval. I checked the code for has\_many. The finder\_sql is constructed the old way, not Arel all the way down. The compromise I can think of is to extend the collection code so it loads all the comments in one batch and insert them into your articles. The other one I can think of is to use AJAX to load the recent comments and have a dedicated controller handle that. It’s probably a better design than trying to load a set of articles and recent comments for all of them, in a single go, then rendering them to the page. Ho-Sheng H. > **[lambda { }](http://ruby-lambda.blogspot.com)** > > lambda { put(self).in(:pipe).and.call() }