[Fixed]-How to do this join query in Django

This can be done in Django, but you will need to restructure your models a little bit differently:

class Product(models.Model):
    name = models.CharField(max_length=50)
    product_rank = models.OneToOneField('ProductRank')

class ProductRank(models.Model):
    rank = models.IntegerField(default=0)

Now, when fetching Product objects, you can following the one-to-one relationship in one query using the select_related() method:

Product.objects.filter([...]).select_related()

This will produce one query that fetches product ranks using a join:

SELECT "example_product"."id", "example_product"."name", "example_product"."product_rank_id", "example_productrank"."id", "example_productrank"."rank" FROM "example_product" INNER JOIN "example_productrank" ON ("example_product"."product_rank_id" = "example_productrank"."id")

I had to move the relationship field between Product and ProductRank to the Product model because it looks like select_related() follows foreign keys in one direction only.