schema.py

class User(models.Model):
    name = models.CharField(max_length=64, null=True)

class Category(models.Model):
    name = models.CharField(max_length=128)
    is_active = models.BoolenField(default=False)
    created_by = models.ForeignKey(
        User, on_delete=models.PROTECT, related_name='category_created_by', null=True)
    
class ProductAttribute(models.Model):
    name = models.CharField(max_length=64)

class Product(models.Model):
    name = models.CharField(max_length=128, db_index=True)
    upc = models.CharField(max_length=32, unique=True)
    stock = models.PositiveIntegerField(default=0)
    
    category = models.ForeignKey(
        Category, on_delete=models.PROTECT, related_name='categories', null=True)
    
    attributes = models.ManyToManyField(ProductAttribute)
    
    created_by = models.ForeignKey(
        User, on_delete=models.PROTECT, related_name='product_created_by', null=True)
    
    def __str__(self):
        return "%s (%s)" % (self.name, ", ".join(attribute.name for attribute in self.attributes.all())