title: Django admin: search for foreign key objects rather than <select>?
body: <p>My model looks like this:</p>
  
  <pre><code>class Asset(models.Model):
      serial_number = models.CharField(max_length=100, unique=True)
      asset_tag = models.CharField(max_length=100, unique=True)
  
  class WorkOrder(models.Model):
      asset = models.ForeignKey(Asset)
  </code></pre>
  
  <p>Essentially, a work order is submitted and then an admin assigns an asset to the work order. The <code>asset_tag</code> field is a barcode that we can scan in.  When editing the work order in the Django admin, by default the <code>asset</code> field is displayed as a <code>&lt;select&gt;</code> widget.  What we want to be able to do is have a search field so that we can scan the asset tag and then search for the right asset in the DB to associate with the work order.</p>
  
  <p>I know you can customize the Django admin foreign key to a hard coded query, but I can't figure out how to get it so it does a search based on a field on the admin page.</p>