Generic Functionality without Generic Relations

2022-04-09

When you have some generic functionality like anything commentable, likable, or upvotable, it’s common to use Generic Relations in Django. The problem with Generic Relations is that they create the relationships at the application level instead of the database level, and that requires a lot of database queries if you want to aggregate content that shares the generic functionality. There is another way that I will show you in this article.

I learned this technique at my first job in 2002 and then rediscovered it again with Django a few years ago. The trick is to have a generic Item model where every other autonomous model has a one-to-one relation to the Item. Moreover, the Item model has an item_type field, allowing you to recognize the backward one-to-one relationship.

Then whenever you need to have some generic categories, you link them to the Item. Whenever you create generic functionality like media gallery, comments, likes, or upvotes, you attach them to the Item. Whenever you need to work with permissions, publishing status, or workflows, you deal with the Item. Whenever you need to create a global search or trash bin, you work with the Item instances.

Let’s have a look at some code.

Items

First, I'll create the items app with two models: the previously mentioned Item and the abstract model ItemBase with the one-to-one relation for various models to inherit:

# items/models.py
import sys

from django.db import models
from django.apps import apps

if "makemigrations" in sys.argv:
    from django.utils.translation import gettext_noop as _
else:
    from django.utils.translation import gettext_lazy as _


class Item(models.Model):
    """
    A generic model for all autonomous models to link to.

    Currently these autonomous models are available:
    - content.Post
    - companies.Company
    - accounts.User
    """
    ITEM_TYPE_CHOICES = (
        ("content.Post", _("Post")),
        ("companies.Company", _("Company")),
        ("accounts.User", _("User")),
    )
    item_type = models.CharField(
        max_length=200, choices=ITEM_TYPE_CHOICES, editable=False, db_index=True
    )

    class Meta:
        verbose_name = _("Item")
        verbose_name_plural = _("Items")

    def __str__(self):
        content_object_title = (
            str(self.content_object) if self.content_object else "BROKEN REFERENCE"
        )
        return (
            f"{content_object_title} ({self.get_item_type_display()})"
        )

    @property
    def content_object(self):
        app_label, model_name = self.item_type.split(".")
        model = apps.get_model(app_label, model_name)
        return model.objects.filter(item=self).first()


class ItemBase(models.Model):
    """
    An abstract model for the autonomous models that will link to the Item.
    """
    item = models.OneToOneField(
        Item,
        verbose_name=_("Item"),
        editable=False,
        blank=True,
        null=True,
        on_delete=models.CASCADE,
        related_name="%(app_label)s_%(class)s",
    )

    class Meta:
        abstract = True

    def save(self, *args, **kwargs):
        if not self.item:
            model = type(self)
            item = Item.objects.create(
                item_type=f"{model._meta.app_label}.{model.__name__}"
            )
            self.item = item
        super().save()

    def delete(self, *args, **kwargs):
        if self.item:
            self.item.delete()
        super().delete(*args, **kwargs)

Then let's create some autonomous models that will have one-to-one relations with the Item. By "autonomous models," I mean those which are enough by themselves, such as posts, companies, or accounts. Models like types, categories, tags, or likes, wouldn't be autonomous.

Posts

Second, I create the content app with the Post model. This model extends ItemBase which will create the one-to-one relation on save, and will define the item_type as content.Post:

# content/models.py
import sys

from django.contrib.auth.base_user import BaseUserManager
from django.db import models
from django.contrib.auth.models import AbstractUser

if "makemigrations" in sys.argv:
    from django.utils.translation import gettext_noop as _
else:
    from django.utils.translation import gettext_lazy as _

from items.models import ItemBase


class Post(ItemBase):
    title = models.CharField(_("Title"), max_length=255)
    slug = models.SlugField(_("Slug"), max_length=255)
    content = models.TextField(_("Content"))

    class Meta:
        verbose_name = _("Post")
        verbose_name_plural = _("Posts")

Companies

Third, I create the companies app with the Company model. This model also extends ItemBase which will create the one-to-one relation on save, and will define the item_type as companies.Company:

# companies/models.py
import sys

from django.contrib.auth.base_user import BaseUserManager
from django.db import models
from django.contrib.auth.models import AbstractUser

if "makemigrations" in sys.argv:
    from django.utils.translation import gettext_noop as _
else:
    from django.utils.translation import gettext_lazy as _

from items.models import ItemBase


class Company(ItemBase):
    name = models.CharField(_("Name"), max_length=255)
    slug = models.SlugField(_("Slug"), max_length=255)
    description = models.TextField(_("Description"))

    class Meta:
        verbose_name = _("Company")
        verbose_name_plural = _("Companies")

Accounts

Fourth, I'll have a more extensive example with the accounts app containing the User model. This model extends AbstractUser from django.contrib.auth as well as ItemBase for the one-to-one relation. The item_type set at the Item model will be accounts.User:

# accounts/models.py
import sys

from django.db import models
from django.contrib.auth.base_user import BaseUserManager
from django.contrib.auth.models import AbstractUser

if "makemigrations" in sys.argv:
    from django.utils.translation import gettext_noop as _
else:
    from django.utils.translation import gettext_lazy as _

from items.models import ItemBase


class UserManager(BaseUserManager):
    def create_user(self, username="", email="", password="", **extra_fields):
        if not email:
            raise ValueError("Enter an email address")
        email = self.normalize_email(email)
        user = self.model(username=username, email=email, **extra_fields)
        user.set_password(password)
        user.save(using=self._db)
        return user

    def create_superuser(self, username="", email="", password=""):
        user = self.create_user(email=email, password=password, username=username)
        user.is_superuser = True
        user.is_staff = True
        user.save(using=self._db)
        return user


class User(AbstractUser, ItemBase):
    # change username to non-editable non-required field
    username = models.CharField(
        _("Username"), max_length=150, editable=False, blank=True
    )
    # change email to unique and required field
    email = models.EmailField(_("Email address"), unique=True)
    bio = models.TextField(_("Bio"))

    USERNAME_FIELD = "email"
    REQUIRED_FIELDS = []

    objects = UserManager()

Creating new items

I will use the Django shell to create several autonomous model instances and the related Items too:

>>> from content.models import Post
>>> from companies.models import Company
>>> from accounts.models import User
>>> from items.models import Item
>>> post = Post.objects.create(
...     title="Hello, World!",
...     slug="hello-world",
...     content="Lorem ipsum…",
... )
>>> company = Company.objects.create(
...     name="Aidas & Co",
...     slug="aidas-co",
...     description="Lorem ipsum…",
... )
>>> user = User.objects.create_user(
...     username="aidas",
...     email="aidas@example.com",
...     password="jdf234oha&6sfhasdfh",
... )
>>> Item.objects.count()
3

Aggregating content from all those relations

Lastly, here is an example of having posts, companies, and users in a single view. For that, we will use the Item queryset with annotations:

from django import forms
from django.db import models
from django.shortcuts import render
from django.utils.translation import gettext, gettext_lazy as _

from .models import Item


class SearchForm(forms.Form):
    q = forms.CharField(label=_("Search"), required=False)


def all_items(request):
    qs = Item.objects.annotate(
        title=models.Case(
            models.When(
                item_type="content.Post", 
                then="content_post__title",
            ),
            models.When(
                item_type="companies.Company", 
                then="companies_company__name",
            ),
            models.When(
                item_type="accounts.User",
                then="accounts_user__email",
            ),
            default=models.Value(gettext("<Untitled>")),
        ),
        description=models.Case(
            models.When(
                item_type="content.Post",
                then="content_post__content",
            ),
            models.When(
                item_type="companies.Company",
                then="companies_company__description",
            ),
            models.When(
                item_type="accounts.User", 
                then="accounts_user__bio",
            ),
            default=models.Value(""),
        ),
    )

    form = SearchForm(data=request.GET, prefix="search")
    if form.is_valid():
        query = form.cleaned_data["q"]
        if query:
            qs = qs.annotate(
                search=SearchVector(
                    "title",
                    "description",
                )
            ).filter(search=query)

    context = {
        "queryset": qs,
        "search_form": form,
    }
    return render(request, "items/all_items.html", context)

Final words

You can have generic functionality and still avoid multiple hits to the database by using the Item one-to-one approach instead of generic relations.

The name of the Item model can be different, and you can even have multiple such models for various purposes, for example, TaggedItem for tags only.

Do you use anything similar in your projects?

Do you see how this approach could be improved?

Let me know in the comments!

Cover picture by Pixabay

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