Left Join to the latest row using django - mysql

Question

I have two tables.
Products [id, name, category]
Sales [product, date, price]
I want to get every Product with the latest row from Sales. If there is no row in sales, I still want to get that product row. The rows are foreign key related.
What is the Django way of doing such a left join?

Answer 1

Probably you can use Subquery:
from django.db.models import Subquery, OuterRef
sales = Sales.objects.filter(product=OuterRef('pk')).order_by('-date')
products = Product.objects.annotate(latest_sale = Subquery(sales.values('price')[:1]))

Answer 2

you can get the Sales object in view and render the object in template. for example you can do this.
models.py
class Course(models.Model):
name = models.CharField(max_length=100)
category = models.CharField(max_length=100)
def __str__(self):
return self.title
class Sales(models.Model):
product = models.ForeignKey(Course, on_delete=models.CASCADE, null=True)
price = models.IntegerField()
date = models.DateTimeField(auto_now_add=True, null=True)
def __str__(self):
return self.product.title
views.py
order_by(-date) gets you the latest item from the database
def sales_view(request):
sales = Sales.objects.all().order_by(-date)
return render(request, 'template.html', {'sales':sales})
Now, in your template you can load the template.
{% for sale in sales %}
{{sale.product.name}}
{{sale.price}}
{{sale.date}}
{% endfor %}
This way you get the product object even if there are no sales object. Hope you get some idea from it.