Multi-table Inheritance and the Django Admin

Django’s admin interface is a great way to be able to interact with your models without having to write any view code, and, within limits, it’s useful in production too. However, it can quickly get very crowded when you register lots of models.

Consider the situation where you are using Django’s multi-table inheritance:

from django.db import models

from model_utils.managers import InheritanceManager

class Sheep(models.Model):
    sheep_id = models.AutoField(primary_key=True)
    tag_id = models.CharField(max_length=32)
    date_of_birth = models.DateField()
    sire = models.ForeignKey('sheep.Ram', blank=True, null=True, related_name='progeny')
    dam = models.ForeignKey('sheep.Ewe', blank=True, null=True, related_name='progeny')

    objects = InheritanceManager()

    class Meta:
        verbose_name_plural = 'sheep'

    def __str__(self):
        return '{}: {}'.format(self._meta.verbose_name, self.tag_id)


class Ram(Sheep):
    sheep = models.OneToOneField(parent_link=True)

    class Meta:
        verbose_name = 'ram'
        verbose_name_plural = 'rams'


class Ewe(Sheep):
    sheep = models.OneToOneField(parent_link=True)

    class Meta:
        verbose_name = 'ewe'
        verbose_name_plural = 'ewes'

Ignore the fact there is no specialisation on those child models: in practice you’d normally have some.

Also note that I’ve manually included the primary key, and the parent link fields. This has been done so that the actual columns in the database match, and in this case will all be sheep_id. This will make writing joins slightly simpler, and avoids the (not specific to Django) ORM anti-pattern of “always have a column named id”.

We can use the models like this, but it might be nice to have all sheep in the one admin changelist, and just allow filtering by subclass model.

First, we’ll put some extra stuff onto the parent model, to make obtaining the subclasses simpler. Some of these will use a new decorator, which creates a class version of the @property decorator.

class classproperty(property):
    def __get__(self, cls, owner):
      return self.fget.__get__(None, owner)()


class Sheep(models.Model):
    # Fields, etc. defined as above.

    @classproperty
    @classmethod
    def SUBCLASS_OBJECT_CHOICES(cls):
        "All known subclasses, keyed by a unique name per class."
        return {
          rel.name: rel.related_model
          for rel in cls._meta.related_objects
          if rel.parent_link
        }

    @classproperty
    @classmethod
    def SUBCLASS_CHOICES(cls):
        "Available subclass choices, with nice names."
        return [
            (name, model._meta.verbose_name)
            for name, model in cls.SUBCLASS_OBJECT_CHOICES.items()
        ]

    @classmethod
    def SUBCLASS(cls, name):
        "Given a subclass name, return the subclass."
        return cls.SUBCLASS_OBJECT_CHOICES.get(name, cls)

Note that we don’t need to enumerate the subclasses: adding a new subclass later in development will automatically add it to these properties, even though in this case it would be unlikely to happen.

From these, we can write some nice neat stuff to enable using these in the admin.

from django import forms
from django.conf.urls import url
from django.contrib import admin
from django.utils.translation import ugettext as _

from .models import Sheep


class SubclassFilter(admin.SimpleListFilter):
    title = _('gender')
    parameter_name = 'gender'

    def lookups(self, request, model_admin):
      return Sheep.SUBCLASS_CHOICES

    def queryset(self, request, queryset):
      if self.value():
        return queryset.exclude(**{self.value(): None})
      return queryset


@admin.register(Sheep)
class SheepAdmin(admin.ModelAdmin):
    list_display = [
        'tag_id',
        'date_of_birth',
        'gender'
    ]
    list_filter = [SubclassFilter]

    def get_queryset(self, request):
      return super(SheepAdmin, self).get_queryset(request).select_subclasses()

    def gender(self, obj):
        return obj._meta.verbose_name

    def get_form(self, request, obj=None, **kwargs):
        if obj is None:
            Model = Sheep.SUBCLASS(request.GET.get('gender'))
        else:
            Model = obj.__class__

        # When we change the selected gender in the create form, we want to reload the page.
        RELOAD_PAGE = "window.location.search='?gender=' + this.value"
        # We should also grab all existing field values, and pass them as query string values.

        class ModelForm(forms.ModelForm):
            if not obj:
                gender = forms.ChoiceField(
                    choices=[('', _('Please select...'))] + Sheep.SUBCLASS_CHOICES,
                    widget=forms.Select(attrs={'onchange': RELOAD_PAGE})
                )

            class Meta:
                model = Model
                exclude = ()

        return ModelForm

    def get_fields(self, request, obj=None):
        # We want gender to be the first field.
        fields = super(SheepAdmin, self).get_fields(request, obj)

        if 'gender' in fields:
            fields.remove('gender')
            fields = ['gender'] + fields

        return fields

    def get_urls(self):
        # We want to install named urls that match the subclass ones, but bounce to the relevant
        # superclass ones (since they should be able to handle rendering the correct form).
        urls = super(SheepAdmin, self).get_urls()
        existing = '{}_{}_'.format(self.model._meta.app_label, self.model._meta.model_name)
        subclass_urls = []
        for name, model in Sheep.SUBCLASS_OBJECT_CHOICES.items():
            opts = model._meta
            replace = '{}_{}_'.format(opts.app_label, opts.model_name)
            subclass_urls.extend([
                url(pattern.regex.pattern, pattern.callback, name=pattern.name.replace(existing, replace))
                for pattern in urls if pattern.name
            ])

        return urls + subclass_urls

Wow. There’s quite a lot going on there, but the summary is:

  • We create a custom filter that filters according to subclass.
  • The .select_subclasses() means that objects are downcast to their subclass when fetched.
  • There is a custom form, that, when in create mode, has a selector for the desired subclass.
  • When the subclass is changed (only on the create form), the page is reloaded. This is required in a situation where there are different fields on each of the subclass models.
  • We register the subclass admin url paths, but use the superclass admin views.

I’ve had ideas about this for some time, and have just started using something like this in development: in my situation, there will be an arbitrary number of subclasses, all of which will have several new fields. The code in this page is extracted (and changed) from those ideas, so may not be completely correct. Corrections welcome.

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