title: Possible to have "polymorphic has_one" relationship in rails?
body: <p>I'd like to do something like this:</p>
  
  <pre><code>Category
  --------
  - id
  - name
  
  Tag
  --------
  - id
  - tag
  
  
  Campaign
  --------
  - id
  - name
  - target (either a tag *or* a category)
  </code></pre>
  
  <p>Is a polymorphic association the answer here? I can't seem to figure out how to use it with has_one :target, :as => :targetable.</p>
  
  <p>Basically, I want Campaign.target to be set to a Tag or a Category (or potentially another model in the future).</p>