title: Sequelize - To define foreign key, should I use references or belongsTo? or both?
body: <p>As far as I know, in sequelize, there are two ways to define foreign key.</p>
  
  <p>First, use <code>references</code> like:</p>
  
  <pre><code>sequelize.define('foo', {
      bar_id: {
          type: 'blahblah',
          references: {
             model: Bar,
             key: 'id'
          }
      }
  });
  </code></pre>
  
  <p>and second, use <code>belongsTo</code> method:</p>
  
  <pre><code>Foo.belongsTo(Bar, { foreignKey: 'bar_id', targetKey: 'id' });
  </code></pre>
  
  <p>Then when I define foreign key in a model, should I use one of them? or both?</p>
  
  <ul>
  <li>If I should use both, what is the difference between them?</li>
  <li>Or if <code>belongsTo</code> is enough for defining foreign key, can I remove the <code>bar_id</code> definition in <code>sequelize.define('foo', {...})</code>?</li>
  </ul>