[PATCH] Double loading associations when using preload and an identity map

I'll start off by saying this bug is not reproducible without the use of an in-process identity map plugin. Given that, the bug is in the way ActiveRecord chooses whether or not to preload associations.

Example failure scenario:
class Author
has_many :posts, :include => [:comments] end

class Post
has_many :comments end

post = Post.find(1)
post.comments.to_a # to trigger load_target
posts = Author.find(post.author_id).posts
posts.to_a # to trigger load_target

If and only if you have a identity map-ish plugin, posts's objects will include post. I don't mean equivalent AR objects, but the objects with the same object_id.

Given the include option of has_many :posts, AR will iterate over and load all the comments for each post. But it only checks to see if the first post has its comments loaded before returning:

"return if records.first.send(reflection.name).loaded?"

So if the first element of record is not the post we loaded prior, then the post object has two copies of each of its comments in its association proxy.

Given that the identity map plugin is not included in AR, I should make clear the purpose of the patch is to correct the assumption made by AR's preloader that the either all or none of the records' associations are loaded.