title: Is there any way to validate a foreign key using Laravel's Validation syntax?
body: <p>Say I have an <code>object</code> which relates to a <code>mission</code>. Users can upload objects and say that it has a relationship with one of these missions. Is there anyway to check that the <code>mission_id</code> that the user has selected for this object actually exists in this database using Laravel's <code>Validation</code> class and rules?</p>
  
  <p>This way, I can check that <code>mission_id</code> is not only an integer, but also exists in the database.</p>
  
  <p>For example, I guess I'm looking for something like:</p>
  
  <pre><code>$rules = array(
      'mission_id' =&gt; 'foreignKeyExistsInMissions|integer'
  )
  </code></pre>
  
  <p>Where <code>foreignKeyExistsInMissions</code> is the validation rule I'm looking for. </p>