title: Query between has_many relationship
body: <p>I have a projects model that has the following</p>
  
  <pre><code>has_many :project_topics
  has_many :topics, :through =&gt; :project_topics
  </code></pre>
  
  <p>I need a way to be able to query a <code>topic id</code> and have it return all the projects that have that topic. Any ideas?</p>
  
  <pre><code>def projects_by_topic(topic)
  end
  </code></pre>