Django Reverse Relationship

Rationale

Have you ever needed to manage a ManyToManyField on both sides of the relationship in the admin and found out that Django does not make this easy?

Example:

Consider the following models:

from django.contrib import admin
from django.db import models

class Topping(models.Model):
    name = models.CharField(max_length=255)

    def __str__(self):
        return self.name


class Pizza(models.Model):
    name = models.CharField(max_length=255)
    toppings = models.ManyToManyField(Topping, related_name="pizzas")

    def __str__(self):
        return self.name


@admin.register(models.Pizza)
class PizzaAdmin(admin.ModelAdmin):
    fields = ["name", "toppings"]
    filter_horizontal = ["toppings"]


@admin.register(models.Topping)
class ToppingAdmin(ReverseRelationshipAdmin):
    fields = ["name", "pizzas"]
    filter_horizontal = ["pizzas"]

In this example, you can easily add the toppings field to PizzaAdmin, but trying to add pizzas to ToppingAdmin will raise the following error:

Unknown field(s) (pizzas) specified for Topping

This package solves this problem by providing the ReverseRelationshipAdmin:

from reverse_relationship.admin import ReverseRelationshipAdmin
from .models import Topping

@admin.register(Topping)
class ToppingAdmin(ReverseRelationshipAdmin):
    fields = ["name"]
    related_fields = ["pizzas"]
    related_filter_horizontal = ["pizzas"]

Customization

The following attributes can be used to customize the ReverseRelationshipAdmin class.

related_querysets

You can customize the queryset for related fields by passing a dictionary to related_querysets, where keys are the field names and values are the QuerySet instances. This allows you to limit the available options in the reverse relationships.

related_querysets = {
    "pizzas": Pizza.objects.filter(name__startswith="veggie"),
}

related_filter_horizontal / related_filter_vertical

Use these options to display reverse fields with Django’s FilteredSelectMultipleField widget, either horizontally or vertically.

related_filter_horizontal = ["pizzas"]

related_labels

You can specify custom labels for reverse relationships.

related_labels = {
    "pizzas": "Available Pizzas",
}

Hooks

Each of these attributes has a hook available (eg. get_related_querysets) than can be overwritten to customize the value based on the request object and/or model instance.

Use Outside of Admin

Use reverse_relationship_form_factory to create model forms with reverse relationships anywhere in your Django app.

from reverse_relationship.forms import reverse_relationship_form_factory 
from .models import Topping

ToppingForm = reverse_relationship_form_factory(
    model=Topping,
    related_fields=["pizzas"],
)

The generated HTML form will look like this:

<div>
    <label for="id_name">Name:</label>
    <input type="text" name="name" maxlength="255" required id="id_name">
</div>
<div>
    <label for="id_pizzas">Pizzas:</label>
    <select name="pizzas" id="id_pizzas" multiple>
        <!-- Whatever pizzas are in your database -->
        <option value="1">Veggie</option>
        <option value="2">Cheese</option>
        <option value="3">Mediterranean</option>
    </select>
</div>

Limitations

While primarily designed for ManyToManyField relationships, this package also supports ForeignKey relationships, but only if the foreign key is nullable.