title: Django incorrectly expecting id column
body: <p>I'm working with an existing SQL database in Django. These tables for some reason were never given primary keys so I'm just going through and assigning them ones. In one of these models, I changed an existing unique index to a primary index using <code>primary_key = True</code>. I then ran <code>./manage.py makemigrations (app_name); ./manage.py migrate</code>. I came across this error: <strong>(1091, "Can't DROP 'id'; check that column/key exists")</strong>. It seems as though Django assumed that there was an id field for the model when there wasn't because I was able to use phpMyAdmin to make an id field, and when I re-ran the migration it succeeded. While I was able to fix my problem I doubt that this is the best way to go about it. What is the correct way to deal with this problem?</p>