title: bookshelf.js relationship hasMany error
body: <p>I'm getting the following error on a <code>hasMany</code> call using Bookshelf:</p>
  
  <pre><code> A valid target model must be defined for the roles hasMany relation
  </code></pre>
  
  <p>Role.js</p>
  
  <pre><code>var Data = require('../server-includes/Data'),
      User = require('./User');
  
  var Role = Data.bookshelf.Model.extend({
    tableName: 'roles',
    users: function() {
      return this.hasMany(User, 'role_id');
    }
  });
  
  module.exports = Role;
  </code></pre>
  
  <p>User.js</p>
  
  <pre><code>var Data = require('../server-includes/Data'),
      Role = require('./Role');
  
  var User = Data.bookshelf.Model.extend({ 
    tableName: 'users',
    role: function() {
      return this.belongsTo(Role, 'id');
    },
  };
  
  module.exports = User;
  </code></pre>
  
  <p>Usage:</p>
  
  <pre><code>new Role({ id: req.params.id })
  .fetch({ require: true, withRelated:['users'] })
  .then(function (role) {
    role.users().fetch().then(function(users) {
      console.log('users: ' + users);
    });
  })
  </code></pre>