title: rails renaming associations
body: <p>I have two models, TreeNode and User.  Each user <code>has_one</code> TreeNode, which is the root of the tree.</p>
  
  <pre><code>class TreeNode
    acts_as_tree
    belongs_to :user
  end
  
  class User
    has_one :tree_node
  end
  </code></pre>
  
  <p>I would like to have this setup so that rails will make the association so that I can do something like</p>
  
  <pre><code>User.first.tree
  </code></pre>
  
  <p>instead of </p>
  
  <pre><code>User.first.tree_node
  </code></pre>
  
  <p>How would one go about doing something like this?</p>