title: Laravel Datatables order by relationship column does not work
body: <p>I have one <code>User</code> who can be assigned to many <code>Company</code>. I'm trying to render a table using Laravel Datatables &amp; jQuery Datatables. It renders nicely and when clicking on the order icon in the table header, it sorts data by that column, except, it doesn't work for the relationship column <code>company_name</code>.
  This is my code in the controller:</p>
  
  <pre><code>$users = User::with(['roles','companies'])
          -&gt;where('users.id', '!=', Auth::id())
          -&gt;whereHas('roles', function($q){$q-&gt;whereId(Role::ROLE_6);});
  </code></pre>
  
  <p>...</p>
  
  <pre><code>return Datatables::of($users)
  -&gt;editColumn('company', function (User $user) {
                  return $user-&gt;hasCompanies()? $user-&gt;companies-&gt;first()-&gt;company_name : trans('lang.company.not_assigned');
              })
  -&gt;orderColumn('company', 'company')
  -&gt;make(true);
  </code></pre>
  
  <p>And this is my javascript for datatables:</p>
  
  <pre><code>otable = $('#datatable_fixed').DataTable({
                  "ajax": {
                      url: 'users/datatable',
                      type: 'POST',
                  },
                  "pageLength": 15,
                  "processing": true,
                  "stateSave": true,
                  "serverSide": true,
                  "bDestroy": true,
                  columns: [
                      {data: 'first_name', name: 'first_name'},
                      {data: 'last_name', name: 'last_name'},
                      {data: 'company', name: 'company.company_name'},
                      {data: 'email', name: 'email'},
                      {data: 'status', name: 'status'},
                  ],
                  dom: 'Bfrtip',
                  searching: false,
                  "order": [[0, 'asc']],
                  "autoWidth": true,
              });
  </code></pre>