How to do it...

Follow these practices when working with models that use models from other apps:

1. For foreign keys and many-to-many relationships with models from other apps, use the "<app_label>.<model>" declaration instead of importing the model. In Django this works with ForeignKey, OneToOneField, and ManyToManyField, for example:

# myproject/apps/ideas/models.py
from django.db import models
from django.conf import settings
from django.utils.translation import gettext_lazy as _

class Idea(models.Model):
    author = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        verbose_name=_("Author"),
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )
    category = models.ForeignKey(
        "categories.Category",
        verbose_name=_("Category"),
        blank=True,
        null=True,
        on_delete=models.SET_NULL,
    )
    # other fields, attributes, properties and methods…

Here, settings.AUTH_USER_MODEL is a setting with a value such as "auth.User":

2. If you need to access a model from another app in a method, import that model inside the method instead of at the module level, for example, as follows:

# myproject/apps/categories/models.py
from django.db import models
from django.utils.translation import gettext_lazy as _

class Category(models.Model):
    # fields, attributes, properties, and methods…

    def get_ideas_without_this_category(self):
        from myproject.apps.ideas.models import Idea
        return Idea.objects.exclude(category=self)

3. If you use model inheritance, for example, for model mixins, keep the base classes in a separate app and place them before other apps that would use them in INSTALLED_APPS, as follows:

# myproject/settings/_base.py

INSTALLED_APPS = [
    # contributed
    "django.contrib.admin",
    "django.contrib.auth",
    "django.contrib.contenttypes",
    "django.contrib.sessions",
    "django.contrib.messages",
    "django.contrib.staticfiles",
    # third-party
    # ...
    # local
    "myproject.apps.core",
    "myproject.apps.categories",
    "myproject.apps.ideas",
]

Here the ideas app will use the model mixins from the core app as follows:

# myproject/apps/ideas/models.py
from django.db import models
from django.conf import settings
from django.utils.translation import gettext_lazy as _

from myproject.apps.core.models import (
    CreationModificationDateBase,
    MetaTagsBase,
    UrlBase,
)

class Idea(CreationModificationDateBase, MetaTagsBase, UrlBase):
    # fields, attributes, properties, and methods…