title: Collection pluck by relation id and get count
body: <p>I have  Project and Country Model. There is a many to many relations. I get projects with countries. Result is below </p>
  
  <pre><code>array:5 [▼
    0 =&gt; array:5 [▼
      "id" =&gt; 2
      "account_id" =&gt; 1
      "start_date" =&gt; "Jul 2012"
      "end_date" =&gt; "Aug 2013"
      "countries" =&gt; array:1 [▼
        0 =&gt; array:2 [▼
          "id" =&gt; 148
          "pivot" =&gt; array:2 [▶]
        ]
      ]
    ]
    1 =&gt; array:5 [▶]
    2 =&gt; array:5 [▶]
    3 =&gt; array:5 [▶]
    4 =&gt; array:5 [▶]
  ]
  </code></pre>
  
  <p>I wont to get all unique countries count. In the now I do it with this way</p>
  
  <pre><code>$projects-&gt;pluck('countries')-&gt;collapse()-&gt;pluck('id')-&gt;unique()-&gt;count()
  </code></pre>
  
  <p>Question. Can I use pluck with nested relation column and has any more shortly and good solution?? for example like this</p>
  
  <pre><code>$projects-&gt;pluck('countries.id')-&gt;count();
  </code></pre>