title: Eloquent - join clause with string value rather than column heading
body: <p>I have a question regarding join clauses in Eloquent, and whether you can join on a string value rather than a table column.</p>
  
  <p>I have the code below querying a nested set joining parent/child records in a table 'destinations' via a table 'taxonomy'.</p>
  
  <p>The second <code>$join</code> statement in the closure is the one causing an issue; Eloquent assumes this is a column, when I would actually just like to join on <code>t1.parent_type = 'Destination'</code> - ie, <code>t1.parent_type</code> should = a string value, <code>Destination</code>. </p>
  
  <pre><code>$result = DB::connection()
      -&gt;table('destinations AS d1')
      -&gt;select(array('d1.title AS level1', 'd2.title AS level2'))
      -&gt;leftJoin('taxonomy AS t1', function($join) {
          $join-&gt;on('t1.parent_id', '=', 'd1.id');
          $join-&gt;on('t1.parent_type', '=', 'Destination');
      })
      -&gt;leftJoin('destinations AS d2', 'd2.id', '=', 't1.child_id')
      -&gt;where('d1.slug', '=', $slug)
      -&gt;get();
  </code></pre>
  
  <p>Is it possible to force Eloquent to do this? I've tried replacing <code>'Destination'</code> with <code>DB::raw('Destination')</code> but this does not work either.</p>
  
  <p>Thanking you kindly. </p>