Re: Building correct queryset

Hello, Дмитрий,
you can try this one, but w/o further optimizations it may be a very slow query.

qs = Product.objects.filter(
    #Where score is greater or equal
    #to the 4th max score from its group
    score__gte=Subquery(
        (Product.objects
            .filter(brand=OuterRef('brand'))
            .values('score')
            .order_by('-score')[3:4]
        )
    )
).order_by('-score')

 

On Friday, May 19, 2017 at 12:32:49 PM UTC+3, Горобец Дмитрий wrote:

Hello. I have these two models. I have to show 4 or less products on a page for each brand ordered by score. Please, help construct correct queryset.


class Brand(models.Model):
    title = models.CharField(
        verbose_name='Название бренда',
        max_length=64,
        unique=True,
        db_index=True,
        error_messages={
            'unique': 'Бренд с таким именем уже существует.'
        }
    )

    slug = AutoSlugField(
        verbose_name='Адрес страницы бренда',
        populate_from='title',
        editable=True,
        unique=True,
        slugify=custom_slugify,
        db_index=True
    )

    owner = models.OneToOneField(
        to=settings.AUTH_USER_MODEL,
        verbose_name='Владелец',
        on_delete=models.PROTECT,
    )

    score = models.DecimalField(
        verbose_name='Рейтинг бренда',
        blank=True,
        null=True,
        max_digits=19,
        decimal_places=18,
    )


class Product(models.Model):
    title = models.CharField(
        verbose_name='Название изделия',
        max_length=255,
        db_index=True,
    )

    slug = AutoSlugField(
        verbose_name='Адрес страницы изделия',
        populate_from='title',
        editable=False,
        unique_with='brand',
        slugify=custom_slugify,
        db_index=True,
    )

    brand = models.ForeignKey(
        to=Brand,
        verbose_name='Бренд',
        related_name='products',
        on_delete=models.CASCADE,
    )

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