title: Django rounding decimal number to n decimal places based on another column
body: <p>So I am using PostgreSQL and Django with the following model</p>
  <pre><code>class Example(model.Model):
      num = models.DecimalField(max_digits=20, decimal_places=10)
      round_to= models.IntegerField()
  </code></pre>
  <p>What I would like to achieve is that:</p>
  <pre><code>Example.objects.annotate(rounded = Round(F('num'),F('round_to'))
  </code></pre>
  <p>But it seems that <code>Round</code> function only allow me to round to integer. According to: <a href="https://docs.djangoproject.com/en/3.0/ref/models/database-functions/#round" rel="nofollow noreferrer">https://docs.djangoproject.com/en/3.0/ref/models/database-functions/#round</a></p>