Symmetrical ManyToMany Filter Horizontal in Django Admin

foo.py

# Based on post from:
# https://snipt.net/chrisdpratt/symmetrical-manytomany-filter-horizontal-in-django-admin/#L-26

"""
When adding a many-to-many (m2m) relationship in Django, you can use a nice
filter-style multiple select widget to manage entries. However, Django only
lets you edit the m2m relationship this way on the forward model. The only
built-in method in Django to edit the reverse relationship in the admin is
through an InlineModelAdmin.

Below is an example of how to create a filtered multiple select for the reverse
relationship, so that editing entries is as easy as in the forward direction.
"""

### pizza/models.py ###

from django.db import models


class Pizza(models.Model):
    name = models.CharField(max_length=50)
    toppings = models.ManyToManyField(Topping, related_name='pizzas')


class Topping(models.Model):
    name = models.CharField(max_length=50)


### pizza/admin.py ###

from django import forms
from django.contrib import admin
from django.utils.translation import ugettext_lazy as _
from django.contrib.admin.widgets import FilteredSelectMultiple

from .models import Pizza, Topping


@admin.register(Pizza)
class PizzaAdmin(admin.ModelAdmin):
    filter_horizonal = ('toppings',)


class ToppingAdminForm(forms.ModelForm):
    class Meta:
        model = Topping

    pizzas = forms.ModelMultipleChoiceField(
        queryset=Pizza.objects.all(),
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Pizzas'),
            is_stacked=False
        )
    )

    def __init__(self, *args, **kwargs):
        super(ToppingAdminForm, self).__init__(*args, **kwargs)

        if self.instance and self.instance.pk:
            self.fields['pizzas'].initial = self.instance.pizzas.all()

    def save(self, commit=True):
        # Ignore the 'commit' parameter and save topping no matter what.
        topping = super(ToppingAdminForm, self).save(commit=False)
        topping.save()
        topping.pizzas = self.cleaned_data['pizzas']
        self.save_m2m()
        return topping


@admin.register(Topping)
class ToppingAdmin(admin.ModelAdmin):
    form = ToppingAdminForm