title: Using acts_as_list on multiple columns
body: <p>I have a possibly unique case where I need a model to have two differing orders depending on the model it is joined to.  Example as follows:</p>
  
  <pre><code>class Book
    acts_as_list :column =&gt; :genre, :scope =&gt; :genre
    acts_as_list :column =&gt; :author, :scope =&gt; :author
    belongs_to :genre
    belongs_to :author
  end
  </code></pre>
  
  <p>So basically what I am trying to do is have a Book model which is part of two lists, one for the genre page it appears on, and one for the author page it appears on.</p>
  
  <p><code>acts_as_list</code> does not appear to support the use of 2 position columns as methods such as <code>move_to_top</code> does not allow you to specify which list to move to the top of.</p>
  
  <p>Has anyone got any suggestions on how I could achieve this?  Right now I am thinking I will have to create a join table such as <code>books_genres</code> which has a <code>position</code> column, but I am really not too keen on that as that requires a whole load of extra tables.</p>