Caching limited associations

Try post.popular_comments.reload

First, limits are in fact ignored when eager loading. From the docs:

If you eager load an association with a specified :limit option, it will be ignored, returning all the associated objects

This means, like you discovered, you have to force the association into the parent object yourself. In my experiments, post.popular_comments didn't work (this makes sense, since it returns a proxy object), and interestingly neither did post.popular_comments.all. post.popular_comments(true) does the trick, however. Underneath that code calls reload, and simply doing post.popular_comments.reload also gets the association loaded into the parent class.

I'm not sure which of these two is more correct, post.popular_comments(true) or post.popular_comments.reload. Both seem a bit brittle, but the second one reads nicer and expresses your intent more clearly.

I validated that both these methods:

My script to store the post:

require 'pp'
Rails.cache.clear
post = Post.first

#post.popular_comments(true)
post.popular_comments.reload

Rails.logger.info "writing to cache"
s = Rails.cache.write "post", post
Rails.logger.info "writing to cache done"

And to retrieve:

require 'pp'
Rails.logger.info "reading from cache"
post = Rails.cache.read "post"
Rails.logger.info "reading from cache done"
Rails.logger.info post.popular_comments.inspect

If I run one after the other my log shows:

  Post Load (0.5ms)  SELECT `posts`.* FROM `posts` LIMIT 1
  Comment Load (0.5ms)  SELECT `comments`.* FROM `comments` WHERE `comments`.`post_id` = 1 ORDER BY votes LIMIT 20
writing to cache
writing to cache done
reading from cache
reading from cache done
[#<Comment id: 1, ...

My mySQL log also confirms that the second script does not trigger a query for the association.

This was done with Rails 3.1.1

I am not seeing the problem that you described in my tests.

Code that I have used in controller

def show
  unless Rails.cache.exist?('faq_category')
    @faq_category = Faq::Category.first
    @faq_category.questions
    Rails.cache.write('faq_category', @faq_category)
  end
  @faq_category = Rails.cache.read('faq_category')
end

When I run the page I see the following statements in log which says that the models are not getting reloaded Larger image available at https://skitch.com/aroop/g9w5t/untitled

The problem could be with your view file. Comment the view and see if the problem is still there.