Problem in updating database models.

[akshat]

I am using Django 1.8 to build a demo project.Inside my project I have one app 'Book'.Book app initially had this model - 

class Book(models.Model):

      title = models.CharField(max_length=200)

      author = models.TextField()

      pub_date = models.DateTimeField('date published')

I applied makemigrations and then filled some data through my frontend website.Now Book model has 3 rows. Now I wanted to add another field to Book model like this  -

class Book(models.Model):

      title = models.CharField(max_length=200)

      author = models.TextField()

      pub_date = models.DateTimeField('date published')
       thumbnail = models.FileField(upload_to=get_upload_file_name)

In fear of losing my existing data I dumped my data in a json file using python manage.py dumpdata command. Now what are the next steps in this?I mean now I want to empty the whole book table.Then apply makemigrations because I added one field.Also will I have to add default to thumbnail field when applying migrations?

After that I want to populate the table again from an existing json file.

Thanks.