@article = Article.new
Creates a new instance of a Model.
@comment = @article.comments.build
Just like the new method, but by assocation proxies (not for Models). It automatically sets the foreign_key.
The other thing is that when you are doing a “has_one” the build looks like this:
@comment = @article.build_comments
I am writing this posts because searching “rails new vs build” on google did not yield any good results. I found my answer with the help of a friend, and through this post here: http://railsforum.com/viewtopic.php?pid=39621#p39621