RuntimeError: Model class mainpage_authentication.models.User doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS

Question

Before applying models to my project everything was okay. But when I just applied them, the system crushed. I'm a freshmen learning Django, so will be grateful for an explanation, and a solution, if possible.

My code:

admin.py

from django.contrib import admin
from .models import User


admin.site.register(User)

apps.py

from django.apps import AppConfig


class AuthenticationPageConfig(AppConfig):
    default_auto_field = 'django.db.models.BigAutoField'
    name = 'mainpage_authentication'

forms.py

from django import forms
from .models import User


class AuthenticationLoginForm(forms.Form):
    class Meta:
        model = User
        fields = ['username', 'password']

models.py

from django.db import models


class User(models.Model):
    username = models.CharField(max_length=100)
    password = models.CharField(max_length=100)

settings.py

INSTALLED_APPS = [
    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'mainpage_authentication',
]

views.py

from django.shortcuts import render
from django import views
from mainpage_authentication.forms import AuthenticationLoginForm


class MainPageView(views.View):
    def get(self, request):
        return render(request, 'mainpage.html')


class AuthenticationLoginPageView(views.View):
    def get(self, request):
        form: AuthenticationLoginForm = AuthenticationLoginForm()
        return render(request, 'authentication-page.html', {"form": form})

Answer 1

You can use

from mainpage_authentication.models import User

instead of

from .models import User

Basically use an absolute import instead of a relative import for that model.