title: Handling PostgreSQL serial field type in South
body: <p>I am using a legacy database which does a couple things that make sense in a db way, but not sure how to represent them in Django so that South and Django itself can deal with them.</p>
  
  <p>I have a Parts table with PartCode as the key
  I have a Vendor table with VendorCode as the key</p>
  
  <p>I have a PartsVendor table with FK's to Parts and Vendor, as well as additional information about the relationship. I am using the "through" parameter so it stands on it's own, but it uses the PartCode+VendorCode as a composite key, something not supported in Django. Only when using South or functions like dumpdata where it wants to see a primary key do I run into an issue. However, those are pretty big issues.</p>
  
  <p>My temporary solution was to just add an _id field as AutoField and added a serial field in Postgres which works fine, but then when using South it chokes on the fact that is default=False and NOT NULL is true.</p>
  
  <p>I've gone down the path of trying to write a custom field, but this seemed like a dead end since I am not actually changing anything about the field type.</p>