title: How to solve not null constraints
body: <p>My models.py file is as following</p>
  
  <pre><code>Class Project(models.Model):
      ABI_choices = (
      ('android-tv/x86','android-tv/x86'),
      ('abi_2', 'google_apis/x86'),
      ('abi_3','google_apis/x86_64'), )
      Screen_size = (
          (1,"Landscape"),
          (2,"Prtarit")
      )
      API_level = (
          ('android-22','Lolipop'),
          ('android-23','kitkat'),
          ('android-25','marshmellow'),
      )
      CPU_delay = (
          (1,0),
          (2,100),
          (3,200),
          (4,300),
          (5,400),
          (6,500),
      )
      Network_delay = (
          (1,20),
          (2,30),
          (3,40),
          (4,50),
          (5,60),
      )
      abi = models.CharField(choices = ABI_choices,max_length=100,default=None)
      screen = models.CharField(choices = Screen_size,max_length=100,default=None)
      version = models.CharField(choices = API_level,max_length=100,default=None)
      GSM = models.CharField(choices = CPU_delay,max_length=100,default=None)
      network_delay = models.CharField(choices = Network_delay,max_length=100,default=None)
  </code></pre>
  
  <p>but when i try to run migration it show me the following error</p>
  
  <blockquote>
    <p>django.db.utils.IntegrityError: NOT NULL constraint failed:
    app_test_project__new.Network_delay</p>
  </blockquote>