old_contents: from django.db import models
  
  from nhs.prescriptions.models import Product
  
  class Patent(models.Model):
      drug        = models.ForeignKey(Product)
      expiry_date = models.DateField()
      start_date  = models.DateField(null=True, blank=True)
                          # Stupid. But you know, they're called patent numbers.
                          # Except they have letters in them.
      number      = models.CharField(max_length=200, null=True, blank=True)
  
new_contents: from django.db import models
  
  from nhs.prescriptions.models import Product
  
  class Patent(models.Model):
      drug        = models.ForeignKey(Product)
      expiry_date = models.DateField()
      start_date  = models.DateField(null=True, blank=True)
                          # Stupid. But you know, they're called patent numbers.
                          # Except they have letters in them.
      number      = models.CharField(max_length=200, null=True, blank=True)
      source      = models.CharField(max_length=200, null=True, blank=True)
  
subject: Add that field in the model