select_related reverse?(!)

[Andreas Bloch]

Okay, I'm a little confused here about using select_ralated...

Can you only run select_related on a table that HAS a foreign key not a table that IS a foreign key?

What if I want to go: 

SELECT * FROM product LEFT JOIN review on product.id = review.product_id LEFT JOIN comments ON product.id = comment.product_id

example models.py

class Product(models.Model):

    name =

    

class Review(models.Model):

    product = models.Foreignkey(Product)

    rating =

class Comments(models.Model):

    product = models.Foreignkey(Product)

    text =

    

In my templates I want to be able to loop through all reviews and comments for each product without hitting the database multiple times...

How can you achieve this?

[alTus]

Hi. select_related is used for folloing foreign keys. So if u have, say, product and its producer field in it u can do smth like: Product.objects.select_related('producer') and in ur templates accessing product.producer won't hit the database.

Notice that here we have a relationship where only one producer can be ralated to product. But you obviously can't do it when you have multiple objects attached to one.

What you really need is prefetch_related:

Producer.objects.prefetch_related('review_set', 'comments_set') will populate product.review_set.all() and product.comments_set.all() for you.

четверг, 1 мая 2014 г., 17:22:52 UTC+4 пользователь Andreas Bloch написал:

[Venkatraman S]

Instead of spoon-feeding you, I would recommend using DDT and check out the query that is generated and whether if it satisfies your use-case. By this, you will know how exactly select_related works and also the number of queries that gets fired in the page and what happens once you use this.

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[Andreas Bloch]

thanks for your input fellas, solved my problem.