title: Foreign key columns and parent model
body: <p>I have a model like this</p>
  <pre><code>class Order(models.Model):
       product = models.ForeignKey(Product, on_delete=models.DO_NOTHING)
  </code></pre>
  <p>In the product model, I save the price of that product, but from time to time the price may change.</p>
  <p>I want when the order is completed and I want to change the product price of that sold product not update the price of the product that is sold in that order. So, the price change should be affected only new orders.</p>
  <p>Thanks in advance!</p>