How to search for duplicates in the DB through the form before adding?

Question

Django

How to check for two fields 'tin' and 'city'? That is, if, when adding a company, there is a 100% match in these two fields, the user will receive a message that the client is booked.

For example:
tin = 1234567890
city = Moscow

tin = 1234567890
city = Sarov

This addition is possible, the ban on booking only if both fields match.

models.py

from django.db import models
import datetime
from django.utils import timezone
import getpass
from django.contrib.auth.models import User

# Create your models here.
class factory(models.Model):
    city = models.CharField('Город', max_length=30)
    name = models.CharField('Наименование организации', max_length=50)
    tin = models.CharField('ИНН', max_length=12, unique=True)
    equipment = models.CharField('Оборудование', max_length=50)
    dealer = models.ForeignKey(User, verbose_name='Дилер', on_delete=models.CASCADE, max_length=50)
    createdate = models.DateField(verbose_name='Дата бронирования', default=(timezone.localtime(timezone.now()).strftime('%Y-%m-%d')), max_length=50)

    def __str__(self):
        return self.name

    class Meta:
        verbose_name = 'Организация'
        verbose_name_plural = 'Организации'

forms.py

from .models import factory
from django.forms import ModelForm, TextInput

class FactoryForm(ModelForm):
    class Meta:
        model = factory
        fields = ['city', 'name', 'tin', 'equipment']

        widgets = {
            "city": TextInput(attrs={
                'class': 'search-city',
                'type': 'text',
                'list': 'city',
                'placeholder': 'Город (Например: Москва)'
            }),
            "name": TextInput(attrs={
                'class': 'search-llcname',
                'type': 'text',
                'placeholder': 'Наименование организации (Например: ООО "ЭИР-ПАРТ")'
            }),
            "tin": TextInput(attrs={
                'class': 'search-tin',
                'type': 'text',
                'maxlength': '12',
                'minlength': '10',
                'placeholder': 'ИНН'
            }),
            "equipment": TextInput(attrs={
                'class': 'search-equipment',
                'type': 'text',
                'placeholder': 'Поставляемое оборудование'
            }),
        }

views.py

from django.shortcuts import render, redirect
from .models import factory
from .forms import FactoryForm

# Create your views here.
def index(request):
    return render(request, 'main/index.html')


def factory(request):
    error = ''
    if request.method == 'POST':
        form = FactoryForm(request.POST)
        if form.is_valid():
            form.instance.dealer = request.user
            form.save()
        else:
            error = 'Ошибка обработки данных полей'
    
    form = FactoryForm()

    data = {
        'form': form,
        'error': error
    }

    return render(request, 'main/factory.html', data)


def myfactory(request):
    return render(request, 'main/myfactory.html')

Answer 1

Check for entry before adding or use get_or_create. It is also desirable to make a UniqueConstraint for these two fields