title: A model in DJANGO with 4 fields, two of them give error:"add a non-nullable field....". my question is why other two fields dont give error
body: <p>here is the full error:</p>
  
  <p>You are trying to add a non-nullable field 'summary' to product without a default; we can't do that (the database needs something to populate existing rows). Please select a fix: 1) Provide a one-off default now (will be set on all existing rows) 2) Quit, and let me add a default in models.py</p>
  
  <p>My first question is why not all fields give me the error even tough they are identical fields? </p>
  
  <p>Second question how can I fix this without typing this: TextField(default="any string") ? </p>
  
  <p>Third question, I am using mysql,If I was using a different db would I get the same error?</p>
  
  <p><strong>models.py</strong></p>
  
  <pre><code>from django.db import models
  
  class Product(models.Model):
      title=models.TextField()
      description=models.TextField()
      price=models.TextField()
      summary=models.TextField()
  </code></pre>
  
  <p><strong>admin.py</strong></p>
  
  <pre><code>from django.contrib import admin
  from .models import Product
  
  admin.site.register(Product)
  </code></pre>
  
  <p><strong>command</strong></p>
  
  <pre><code>python manage.py makemigrations
  </code></pre>
  
  <p>my admin page has "Product" tab. When I create a product in admin window, i have only two fields. description and price. </p>