Django class choice field

Django class choice field is a utility library designed to make it easy to use one of a choice of classes as a field on a model. It can be used to encapsulate logic associated with a type of the model, without using a polymorphic solution.

from django.db import models
from class_choice_field import ClassChoiceField

class BlogPost(models.Model):
    title = models.CharField()
    status = ClassChoiceField([
        'blog.statuses.Draft',
        'blog.statuses.Approval',
        'blog.statuses.Live',
    ])

Rationale

A common pattern for dealing with having different "flavours" of a model instance is to specify the choices as class attributes and then treat code conditionally depending on the value of the field.

from django.db import models

class BlogPost(models.Model):
    STATUS_DRAFT = 'draft'
    STATUS_APPROVAL = 'approval'
    STATUS_LIVE = 'live'
    STATUS_CHOICES = (
        (STATUS_DRAFT, 'Draft'),
        (STATUS_APPROVAL, 'Approval'),
        (STATUS_LIVE, 'Live'),
    )
    title = models.CharField()
    status = models.CharField(choices=STATUS_CHOICES)

This pattern is great when you have a small number of simple places where the logic changes depending on the status of the object. However on occasions you can have a lot of variation in the associated logic. In the example above, the status could be used to change the rendered template, the associated edit form, the permissions of who can view the object, the permissions of who can edit the object… the list goes on. Instead of littering the code base with switches based on the status, we can instead encapsulate all that logic into a Status object and choose between them.

class Draft(ClassChoice):
    ...

class BlogPost(models.Model):
    title = models.CharField()
    status = ClassChoiceField([Draft])

    objects = models.Manager.from_queryset(status.queryset)

BlogPost.objects.filter(status='draft')
BlogPost.objects.filter(status=Draft)
BlogPost.objects.filter_draft()

post = BlogPost.objects.filter_draft()[0]
post.is_draft
post.status == 'draft'
post.status == Draft

post.status.template_name
post.status.form_class
post.status.has_edit_permission(user)

mjtamlyn/django-class-choice-field

Django class choice field

Rationale