title: Laravel datatable sorting issue
body: <p>I'm using Laravel - data tables by yajra
  <a href="https://github.com/yajra/laravel-datatables-docs" rel="nofollow noreferrer">https://github.com/yajra/laravel-datatables-docs</a></p>
  
  <p>It works perfectly with a single table, but things get serious when I use the eloquent relationship with it. As described below in code I'm showing user name in the table,
  it showing user name perfectly fine but when I try to sort on User or simply search it gives me a wrong error or shows SQL error.</p>
  
  <p>I have following eloquent models</p>
  
  <pre><code>class Project extends Model{
       public function client(){
        return $this-&gt;belongsTo(Client::class);
       }
  }
  </code></pre>
  
  <pre><code>class Client extends Model{
       public function user(){
            return $this-&gt;belongsTo(User::class);
       }
  }
  </code></pre>
  
  <pre><code>class User extends Model{
  }
  </code></pre>
  
  <p>HTML </p>
  
  <pre><code>&lt;table id="table"&gt;
      &lt;tbody&gt;
          &lt;tr&gt;
              &lt;td&gt;Name&lt;/td&gt;
              &lt;td&gt;Start Date&lt;/td&gt;
              &lt;td&gt;Target&lt;/td&gt;
              &lt;td&gt;User&lt;/td&gt;
          &lt;/tr&gt;
      &lt;/tbody&gt;
  &lt;/table&gt;
  
  Javascript
  
  $("#table").DataTable({
          processing: true,
          serverSide: true,
          autoWidth:false,
          ajax: '/projects',
          columns:[
              { data: 'project_name', name: 'project_name' },
              { data: 'start_date', name: 'start_date' },
              { data: 'target', name: 'target' },
              { data: 'client.user.name', name: 'client.user.name' }
          ]
      });
  </code></pre>
  
  <p>Project Controller</p>
  
  <pre><code>public function projects(){
      return Datatables::of(Proejct::with(['client.user']))
              -&gt;addColumn("client.user.name", function($row){
                  return $row-&gt;client-&gt;user-&gt;name;
              })-&gt;make(true);
  }
  </code></pre>