test_models.py

import pytest

from product.models import Category, Product

@pytest.fixture
def product_one(db):
    category = Category.objects.create(name="Books")
    return Product.objects.create(name="Book 1", category=category)

@pytest.fixture
def product_two(db):
    category = Category.objects.create(name="Books")
    return Product.objects.create(name="Book 2", category=category)

def test_two_different_books_create(product_one, product_two):
    assert product_one.pk != product_two.pk