title: Rails conditional association on instance itself
body: <p>Let's say we have a <code>Comment</code> that is connected to an <code>Article</code> through a belongs_to association. Comment has an attribute called <code>is_connected</code> which is a boolean.</p>
  
  <p>I know that it's possible to add a condition to the belongs_to relation. Is it possible to add a condition on the instance itself?</p>
  
  <pre><code>class Comment
    belongs_to :article, -&gt; {where: is_connected: true}
  end
  </code></pre>
  
  <p>Like in this example - is_connected is referencing the article model. But I want the relation to only exist if <code>comment.is_connected == true</code></p>