title: Django: Generate slug field data for existing entries in database
body: <p>Some time ago I created a Django model:</p>
  
  <pre><code>class Product(models.Model):
      name = models.CharField(verbose_name=_('Nome'), max_length=100)
      description = models.CharField(verbose_name=_('Descrizione'), blank=True, default="")
  </code></pre>
  
  <p>Now I would like to insert a slug field:</p>
  
  <pre><code>class Product(models.Model):
      name = models.CharField(verbose_name=_('Nome'), max_length=100)
      slug = AutoSlugField(populate_from='name', unique=True)
      description = models.CharField(verbose_name=_('Descrizione'), blank=True, default="")
  </code></pre>
  
  <p>My problem is that when I create a migration, Django asks me to insert a default value for the slug field.</p>
  
  <p>My idea is to generate a slug during the migration for existing entries in database, there is a way to do it?</p>
  
  <p>Thanks!</p>