Rails - Aug 2010 - how to ? Rails 3 ActiveRecord eager loading and AREL

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
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On Aug 5, 9:48 am, M- Ryan <li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
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
> 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
> --
> Posted viahttp://www.ruby-forum.com/.
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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
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On Aug 6, 3:03 am, M- Ryan <li...-fsXkhYbjdPsEEoCn2XhGlw@public.gmane.org>
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 Hsiao
http://ruby-lambda.blogspot.com

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