Bookshelf.js polymorphic problem

taggable.js

var bookshelf = require('./bookshelf');
var Post = require('./post');

/*
Schema:

posts:
  id: PRIMARY
  title: VARCHAR

tags:
  id: PRIMARY
  tag: VARCHAR

taggable:
  id: PRIMARY
  tag_id: UNSIGNED INT
  taggable_id: UNSIGNED INT
  taggable_type: VARCHAR
*/

var bookshelf = require('./bookshelf');
var Tag = require('./tag');
var Post = require('./post');

var Taggable = bookshelf.Model.extend({
    tableName: 'taggables',
    taggable: function() {
        return this.morphTo('taggable', Post);
    }
});

// How do I specify that taggable morphs tags?