title: Search by foreign key id in admin
body: <p>I'm trying to achieve something apparently simple but I couldn't find any answer, neither on Google nor here. I have a Django model, something dead-simple:</p>
  
  <pre><code>class Shipment(models.Model):
      id = models.AutoField(primary_key=True)
      transaction = models.ForeignKey(Transaction)
  </code></pre>
  
  <p>I would like to be able to search in my Shipment Admin page by transaction.id. To clarity,
  I want this (this code obviously doesn't work):</p>
  
  <pre><code>class ShipmentAdmin(admin.ModelAdmin):
      list_display = ('id', 'transaction')
      search_fields = ['id', 'transaction.id']
  </code></pre>
  
  <p>This can't work cause transaction.id does not name a field. Any idea?
  By "search" I mean be able to insert my transaction id inside the search box of the Shipment Admin page, press "search" and automatically retrieve appropriate transactions.</p>