Django product variation model – how to get distinct color

I am building a ecommerse store with django. One of the requirement is that a product can have a color and different sizes. I also want to keep track of the quantity of each size.
I can create ProductVariation like product=’test’, size=’small’, color=’red’, quantity=5 , product=’test’, size=’large’, color=’red’, quantity=10. Now in db there are two objects created,how can I query that I only get color red in queryset when I am rendering product details.

I looked at this example – Django Products model for e-commerce site
but did not understand this bit –

Color.objects.filter(productvariant__product=some_product).distinct()

These are my models:

class Product(models.Model):
    """
    Product model to allow creating product records and linking with category 
    """
    product_uuid = models.UUIDField(
        default=uuid.uuid4, editable=False, unique=True)
    product_name = models.CharField(max_length=255)
    sub_category = models.ForeignKey(
        'SubCategory', on_delete=models.CASCADE, related_name='products')
    product_description = models.TextField()
    is_featured = models.BooleanField(default=False)
    product_sku = models.CharField(max_length=255, null=True, blank=True)
    product_price = models.DecimalField(max_digits=6, decimal_places=2)

    def __str__(self):
        return self.product_name

    class Meta:
        verbose_name = 'Product'
        verbose_name_plural = 'Products'

class ProductVariation(models.Model):
    """
    ProductVariation model to allow creating product with different size and color
    """
    product_var_uuid = models.UUIDField(
        default=uuid.uuid4, editable=False, unique=True)
    product = models.ForeignKey(
        'Product', on_delete=models.CASCADE, related_name='product_variations')
    size = models.ForeignKey('Size', on_delete=models.CASCADE, related_name="product_sizes")
    color = models.ForeignKey('Color', on_delete=models.CASCADE, related_name="color")
    quantity = models .IntegerField()

    def __str__(self):
        return self.product.product_name

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=['product', 'color', 'size'],
                name='unique_prod_color_size_combo'
            )
        ]
        verbose_name = 'Product Variation'
        verbose_name_plural = 'Product Variations'


class Size(models.Model):
    product_size_uuid = models.UUIDField(
        default=uuid.uuid4, editable=False, unique=True)
    product_size = models.CharField(max_length=20)

    def __str__(self):
        return self.product_size

    class Meta:
        verbose_name = 'Product Size'
        verbose_name_plural = 'Product Sizes'

class Color(models.Model):
    product_color_uuid = models.UUIDField(
        default=uuid.uuid4, editable=False, unique=True)
    product_color = models.CharField(max_length=20)

    def __str__(self):
        return self.product_color

    class Meta:
        verbose_name = 'Product Color'
        verbose_name_plural = 'Product Colors'

>Solution :

What we do here is filter by joining on the ProductVariant and then on the Product, so with a query:

Color.objects.filter(color__product=some_product).distinct()

but this is because the related_name of color in ProductVariation is color which makes not much sense. That should be something like:

class ProductVariation(models.Model):
    # …
    color = models.ForeignKey(
        'Color', on_delete=models.CASCADE, related_name='product_variants'
    )
    # …

since it is the name of the relation in reverse. In that case we filter with:

Color.objects.filter(product_variants__product=some_product).distinct()

This will then make a query that looks like:

SELECT DISTINCT color.*
FROM color
INNER JOIN productvariation ON color.id = productvariation.color_id
WHERE productvariation.product_id = some_product_id