title: Django migrations : relation already exists
body: <p>I have trouble with django model migrations. 
  I have some models in my app, and I already have some data inside. 
  When I added some models in my application, and I run <code>makemigrations</code>, the app report that there is no change. 
  I know that sometimes some errors came when migrate, so I delete django_migrations table in my database and run <code>makemigrations</code> again, and now program found my new fields. </p>
  
  <p>The problem now is that if I run <code>migrate</code> system tell me that some tables already exist. (Which is ok and correct, because they do). I don't want to delete those tables, because I have data already inside.</p>
  
  <p>I can't run <code>migrate --fake</code>, because program will think that I already have <strong>all</strong> the tables, which is not true. </p>
  
  <p>So, I am looking for a way to tell the program : run migration, if table exist skip it. (<code>--fake</code> it)</p>
  
  <p>Another question is why is this happening to me, that makemigrations don't recognise my changes (some cache problems,...)?</p>