title: How to set default value for new model field in existing model with data
body: <p>I have a small django model with two fields. There is already data in the database for this model.</p>
  
  <pre><code>class MetaDataValue(Model):
      metadata_id = models.ForeignKey(MetaData, on_delete=models.CASCADE,)
      value = models.CharField('value', max_length=200,)
  </code></pre>
  
  <p>I need to add another field, </p>
  
  <pre><code>short_value = models.CharField('short_value', max_length=200,)
  </code></pre>
  
  <p>I know when I perform a migration, it will complain because I don't have a default value for the existing rows in the database. </p>
  
  <p>Is there a way to set the default value for <code>short_value</code> to the string in the <code>value</code> field that already exists in the database/model? </p>
  
  <p>I want to do this because I only need to create a different <code>short_value</code> for about 20 rows in the database, and there is no universal default value for this field. I would rather not have something like 'fred' or 'default' in the <code>short_value</code> field because some fields have numbers, some have text, some have a combination of numbers and text. I also thought of creating a property instead of another model field, but there isn't a simple way to convert the <code>value</code> field into the <code>short_value</code> field.</p>
  
  <p>Thanks!</p>