Django choices code example

Example 1: on_delete options django

There are seven possible actions to take when such event occurs:

CASCADE: When the referenced object is deleted, also delete the objects that have references to it 
  (when you remove a blog post for instance, you might want to delete comments as well). 
  SQL equivalent: CASCADE.
      
PROTECT: Forbid the deletion of the referenced object. 
  To delete it you will have to delete all objects that reference it manually. 
  SQL equivalent: RESTRICT.
    
RESTRICT: (introduced in Django 3.1) Similar behavior as PROTECT that matches SQL's RESTRICT more accurately. (See django documentation example)
SET_NULL: Set the reference to NULL (requires the field to be nullable). 
  For instance, when you delete a User, you might want to keep the comments he posted on blog posts, 
  but say it was posted by an anonymous (or deleted) user. 
  SQL equivalent: SET NULL.
    
SET_DEFAULT: Set the default value. SQL equivalent: SET DEFAULT.
    
SET(...): Set a given value. This one is not part of the SQL standard and is entirely handled by Django.
  
DO_NOTHING: Probably a very bad idea since this would create integrity issues in your database 
  (referencing an object that actually doesn't exist). SQL equivalent: NO ACTION.

Example 2: field.choices django

Will create a select box for this in the model form. Use can select from the
options.The first element in each tuple is the actual value to be set on the model, 
and the second element is the human-readable name. For example:
YEAR_IN_SCHOOL_CHOICES = [
        (FRESHMAN, 'Freshman'),
        (SOPHOMORE, 'Sophomore'),
        (JUNIOR, 'Junior'),
        (SENIOR, 'Senior'),
        (GRADUATE, 'Graduate'),
    ]
    year_in_school = models.CharField(
        max_length=2,
        choices=YEAR_IN_SCHOOL_CHOICES,
        default=FRESHMAN,
    )

Example 3: django models choices example

from django.db import models

class Author(models.Model):
   first_name = models.CharField(max_length=64)
   last_name = models.CharField(max_length=64)

   def __str__(self):
       return '{} {}'.format(self.first_name, self.last_name)


class Book(models.Model):
   author = models.ForeignKey('books.Author', related_name='books', on_delete=models.CASCADE)
   title = models.CharField(max_length=128)
   status = models.CharField(
       max_length=32,
       choices=[],  # some list of choices
   )

   def __str__(self):
       return '{}: {}'.format(self.author, self.title)