How to stop Django inserting null columns for NoSql DB

Question

I user django and mongodb for my project. I want to stop django orm from inserting null columns to DB.

Here is some of my django files:

# models.py

from django.db import models
class Ad(models.Model):
    user_id = models.IntegerField()
    title = models.CharField(max_length=100)
    description = models.CharField(max_length=500)
    fee = models.CharField(max_length=20)
    phone = models.CharField(max_length=10)
    location = models.CharField(max_length=50, default="")
    category = models.CharField(max_length=25)
    est_rooms = models.CharField(max_length=10)
    est_area = models.CharField(max_length=10)
    est_age = models.CharField(max_length=10)


# views.py
a = Ad.objects.create(user_id=request.user.id, title=title, description=description, fee=fee, phone=phone, location=location, category=category, est_rooms=est_rooms, est_age=None)
a.save()

For example, if I insert as above although I did not give anything for est_area, it inserts est_area="", but I don't want this. Instead, I want django orm not to insert est_area property. Could anyone help me?