title: Django re-calculate column on model anytime a new model is created or an existing is saved
body: <p>I have a model that has an aggregate field on it. I'd like to recalculate the agg field whenever the model is updated, or calculate it initially if the model is being created.</p>
  <p>model (Just as an example):</p>
  <pre><code>class FooBar(models.Model):
     id = models.BigAutoField(primary_key=True)
     name = models.TextField(blank=True, null=True)
     last_active_date = models.DateTimeField(blank=True, null=True)
  
     agg_field = models.DateTimeField(blank=True, null=True)
  </code></pre>
  <p>Where the agg_field should be a method such as (This could be anything, just wrote this as an example):</p>
  <pre><code>FooBar.objects.filter('last_active_date__gt': datetime.now() - timedelta(days=1)).annotate(Count('pk')))
  </code></pre>
  <p>So essentially every time the model is updated or created, it should recalculate this column</p>