title: Laravel validation: exists with additional column condition - custom validation rule
body: <p>Is there is a way of referencing another field when specifying the exists validation rule in Laravel? I want to be able to say that input a must exist in table a, input b must exist in table b AND the value for column x in table b must equal input a.</p>
  
  <p>Best explained by example:</p>
  
  <pre><code>public $rules = array(
      'game_id' =&gt; 'required|exists:games,id',
      'team1_id' =&gt; 'required|exists:teams,id,game_id,&lt;game_id input value here&gt;',
      'team2_id' =&gt; 'required|exists:teams,id,game_id,&lt;game_id input value here&gt;'
  );
  </code></pre>
  
  <p>So with my validation rules I want to be able to make sure that:</p>
  
  <ul>
  <li><code>game_id</code> exists within the <code>games</code> table (<code>id</code> field)</li>
  <li><code>team1_id</code> exists within the <code>teams</code> table (<code>id</code> field) and the <code>game_id</code> column (in the <code>teams</code> table) must equal the value of the <code>game_id</code> input.</li>
  <li>As above for <code>team2_id</code></li>
  </ul>
  
  <p>So, if in my form, I entered <code>1</code> for <code>game_id</code>, I want to be able to ensure that the record within the teams table for both <code>team1_id</code> and <code>team2_id</code> have the value <code>1</code> for <code>game_id</code>.</p>
  
  <p>I hope this makes sense.</p>
  
  <p>Thanks</p>