Eager loading associations on ActiveModel instances in Rails

Question

In RoR, it is pretty common mistake for new people to load a class and assiocations like this# the solution to eager load

# The bellow generates an insane amount of queries
# post has many comments
# If you have 10 posts with 5 comments each
# this will run 11 queries 
posts = Post.find(:all)
posts.each do |post|
  post.comments
end

The solution is pretty simple to eager load

# should be 2 queries
# no matter how many posts you have
posts = Post.find(:all, :include => :comments) # runs a query to get all the comments for all the posts
posts.each do |post|
  post.comments # runs a query to get the comments for that post
end

But what if you don't have access to the class methods, and only have access to a collection of instance methods.

Then you are stuck with the query intensive lazy loading.

Is there a way to minimize queries to get all the comments for the collection of posts, from the collection of instances?

Addition for Answer (also added to the code above)

---

So to eager load from what I can see in the rdoc for rails is a class method on any extension of ActiveRecord::Associations, the problem is say you you don't have the ability to use a class method, so you need to use some sort of instance method

a code example of what I think it would look like would be is something like

post = Posts.find(:all)
posts.get_all(:comments) # runs the query to build comments into each post without the class method.

Answer 1

In Rails 3.0 and earlier you can do:

Post.send :preload_associations, posts, :comments

You can pass arrays or hashes of association names like you can to include:

Post.send :preload_associations, posts, :comments => :users

In Rails 3.1 this has been moved and you use the Preloader like this:

ActiveRecord::Associations::Preloader.new(posts, :comments).run()

And since Rails 4 its invocation has changed to:

ActiveRecord::Associations::Preloader.new.preload(posts, :comments)