Re: select_related() and RawQuerySet

On Friday, March 2, 2012 7:24:00 PM UTC+2, Tom Evans wrote:

Hi all

I have a particular query that requires me to use a RawQuerySet - I
need to left join the table to itself in order to select the highest
'priority' row from the table for each distinct value of a foreign key
on the model, and I need to join to the related table in order to
filter the results.

Having generated the queryset, I then want to make a dictionary of {
foreign_key_object : model_object }. So I have this working, but I
cannot use select_related() with a RawQuerySet, and so this runs N+1
queries, where N is the number of distinct foreign keys.

Here is some code, which might explain it better:

connection.queries=[]
base_products_qs = Product.objects.raw(
    """
    SELECT idp_product.*, idp_productclass.*
    FROM idp_product
    JOIN idp_productclass
        ON idp_product.product_class_id = idp_productclass.id
    LEFT JOIN idp_product p2
        ON idp_product.product_class_id = p2.product_class_id
            AND p2.class_priority < idp_product.class_priority
    WHERE p2.id IS NULL and idp_productclass.product_type != 4
    """)
base_products = dict([ (p.product_class, p) for p in base_products_qs ])
len(connection.queries) # 7 queries (6 product classes)

Is there any simple way around this? I can reduce it to two queries
already, but it seems wrong to select out the info I want, throw it
away, and then fetch it again.

 Some suggestions:
  - If the 2-query version is fast enough, use it. Premature optimization and all that... :)
  - You could of course manually iterate through the SQL and instantiate the models by hand.
  - Beware of the left join, it seems if you have a lot of rows with different class_priorities for each product_class_id you might be in trouble. The filtering on IS NULL happens after doing the join, so before the filter you could have a lot of intermediate rows. It might be there is no problem depending of the amount of rows and SQL vendor. Hard to know without testing. There are a couple of other ways to do the query, either by subquery doing a select min(class_priority), product_class_id or if you are using PostgreSQL, distinct on would do the job for you, too.

All that being said, I would really like the ability to have select_related available for RawQuerySet. It might be judged to be a feature not needed commonly enough, so it is possible it would not get in even with a good quality patch. However, if it had a nice API, and the implementation reused the code of QuerySet select_related, I think it could have a chance to get in.

 - Anssi

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