test_models.py

import pytest
from product.models import Category, Product, Retail
...

@pytest.fixture
def retailer_abc(db):
    return Retail.objects.create(name="ABC")

@pytest.fixture
def retailers(db) -> list:
    return []

@pytest.fixture(autouse=True)
def append_retailers(retailers, retailer_abc):
    return retailers.append(retailer_abc)

@pytest.fixture
def product_one(db, category, retailers):
    product = Product.objects.create(name="Book 1", category=category)
    product.retails.set(retailers)
    return product

def test_product_retailer(db, retailer_abc, product_one):
    assert product_one.retails.filter(name=retailer_abc.name).exists()