title: Update column in Django with child columns
body: <p>I have 2 models <code>Parent</code>, <code>Child</code></p>
  <pre><code>class Parent(models.Model):
      id = Base64UUIDField(primary_key=True, editable=False)
      cost = models.DateTimeField(default=None, blank=True, null=True)
  
  class Child(models.Model):
      id = Base64UUIDField(primary_key=True, editable=False)
      cost = models.DateTimeField(default=None, blank=True, null=True)
      parent = models.ForeignKey(Parent, related_name= &quot;children&quot;, related_query_name= &quot;child&quot;)
  </code></pre>
  <p>I need to populate <code>cost</code> column of <code>Parent</code> objects to maximum cost of all children of that parent</p>
  <p>I have tried to annotate to a new column <code>new_cost</code>, and its works.</p>
  <pre><code>parents.annotate(new_cost=Max('child__cost'))
  </code></pre>
  <p>But I need to populate values to existing column <code>cost</code>. Tried something like this, but not working.</p>
  <pre><code>parents.update(cost=Max('child__cost'))
  </code></pre>