title: I have only one primary key in my Django Model, but it shows error as multiple primary key
body: <p>My model is</p>
  
  <pre><code>class authenticate(models.Model):
      name=models.CharField(max_length=50)
      uid=models.CharField(max_length=39,editable=False)
      image=models.CharField(max_length=250)
      email=models.CharField(max_length=100,primary_key=True)
      mobile=models.CharField(max_length=12)
      dob=models.CharField(max_length=12)
      regno=models.CharField(max_length=10,editable=False)
      created=models.CharField(max_length=10,editable=False)
      last_login=models.CharField(max_length=10)
      department=models.CharField(max_length=5)
      admin=models.BooleanField(default=False)
  </code></pre>
  
  <p>Error occurred when tried to migrate my model in Heroku repo:</p>
  
  <pre><code>psycopg2.errors.InvalidTableDefinition: multiple primary keys for table "Auth_authenticate" are not allowed
  
  
  The above exception was the direct cause of the following exception:
  
  --
  --
  --
      return self.cursor.execute(sql, params)
    File "/app/.heroku/python/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
      raise dj_exc_value.with_traceback(traceback) from exc_value
    File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/utils.py", line 84, in _execute
      return self.cursor.execute(sql, params)
  django.db.utils.ProgrammingError: multiple primary keys for table "Auth_authenticate" are not allowed
  </code></pre>
  
  <p>The above model is working perfectly when I am using sqlite DB.</p>