title: In a belongs_to has_many relationship in Rails, how does the 'singlular' side access records from the 'many' side?
body: <p>It was my understanding that all keys are stored on the 'many' side, ie: if a user has many posts, and posts belong to only one user, foreign keys associating which user "owns" which which post exist only in the post object.</p>
  
  <p>Yet, in my users controller, I can still create a user show page which lists all posts by looping through 'user.posts'.</p>
  
  <p>How is this possible?</p>
  
  <p>Thanks in advance.</p>
  
  <p>Edit: one caveat, I'm using MongoDB / Mongoid, not sure how this pans out with Active Record.</p>