In django models say this model exist in details/models.py

    class OccDetails(models.Model):
       title = models.CharField(max_length = 255)
       occ = models.ForeignKey(Occ)

So when sync db is made the following fields get created

and later to this of two more fields are added and sync db is made the new fields doesnt get created.How is this to be solved,Also what is auto_now=true in the below

these are the new fields

         created_date = models.DateTimeField(auto_now_add=True)
         modified_date = models.DateTimeField(auto_now_add=True, auto_now=True)