title: Django database migration: merging two fields into one
body: <p>Say I have a model that represents a product from my company. Each product a product number (ex. 5928523), as well as a boolean representing whether or not it is an experimental product.</p>
  
  <pre><code>class Product(models.Model):
  
      product_number = models.IntegerField()
      experimental = models.BooleanField(default=False)
  </code></pre>
  
  <p>Say I want to merge these two fields into one character field, a product ID. So an experimental product with product number 3892 would turn into "X3892" and a non-experimental product with product number 937 would simply turn into "937".</p>
  
  <pre><code>class Product(models.Model):
  
      product_id = models.CharField()
  </code></pre>
  
  <p>How would I write a database migration in Django to achieve this?</p>