0-howto-listfield-django-admin.rst

Howto use ListFields in Django's admin

Problem

Consider this blog post model:

models.py

from django.db import models
from djangotoolbox.fields import ListField

class Post(models.Model):
    title = models.CharField(max_length=100)
    categories = ListField()

admin.py

from django.contrib.admin import site
from models import Post

site.register(Post)

Trying to edit some posts in the admin crashes with:

No form field implemented for <class 'djangotoolbox.fields.ListField'>

Solution

What we need to do is to teach the admin how to display a ListField in the edit view. Our form field will be a simple <input type=text> input box with comma-separated category names. For more about custom form fields, refer to the Django documentation and your favourite search engine using the terms "Django custom form field".

First, we need to subclass ListField to override the formfield method:

from .forms import StringListField

class CategoryField(ListField):
    def formfield(self, **kwargs):
        return models.Field.formfield(self, StringListField, **kwargs)

class Post(models.Model):
    title = models.CharField(max_length=100)
    categories = CategoryField()

Then, in forms.py, we define StringListField:

from django import forms

class StringListField(forms.CharField):
    def prepare_value(self, value):
        return ', '.join(value)

    def to_python(self, value):
        if not value:
            return []
        return [item.strip() for item in value.split(',')]

This will covert the comma-separated input box contents into a Python list, and the list value that is fetched from the database int a comma-separated string which is then displayed in the input box.

Let's add a post and check out the resulting model object in the database:

>>> Post.objects.get(title='foo').categories
[u'spam', u'eggs', u'bar']

It worked! Simple, isn't it?

1-models.py

from django.db import models
from djangotoolbox.fields import ListField
from .forms import StringListField

class CategoryField(ListField):
    def formfield(self, **kwargs):
        return models.Field.formfield(self, StringListField, **kwargs)

class Post(models.Model):
    title = models.CharField(max_length=100)
    categories = CategoryField()

2-forms.py

from django import forms

class StringListField(forms.CharField):
    def prepare_value(self, value):
        return ', '.join(value)

    def to_python(self, value):
        if not value:
            return []
        return [item.strip() for item in value.split(',')]

3-admin.py

from django.contrib.admin import site, ModelAdmin
from models import Post

def categories(instance):
    return ', '.join(instance.categories)

class PostAdmin(ModelAdmin):
    list_display = ['title', categories]

site.register(Post, PostAdmin)